feat(comments): add tap-to-reply UI for nested comments

- Pass postCid to loadComments for reply reference support
- Add onReplyTap callback to CommentThread and CommentCard
- Tapping reply icon on a comment navigates to ReplyScreen
- ReplyScreen receives parent comment for nested replies
- Show "Replying to @handle" context in reply screen

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

lib/screens/home/post_detail_screen.dart

···
   void _loadComments() {
     context.read<CommentsProvider>().loadComments(
       postUri: widget.post.post.uri,
+      postCid: widget.post.post.cid,
       refresh: true,
     );
   }
···
     );
   }
 
-  /// Handle comment submission
+  /// Handle comment submission (reply to post)
   Future<void> _handleCommentSubmit(String content) async {
-    // TODO: Implement comment creation via atProto
-    ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(
-        content: Text('Comment submitted: $content'),
-        behavior: SnackBarBehavior.floating,
-        duration: const Duration(seconds: 2),
+    final commentsProvider = context.read<CommentsProvider>();
+    final messenger = ScaffoldMessenger.of(context);
+
+    try {
+      await commentsProvider.createComment(content: content);
+
+      if (mounted) {
+        messenger.showSnackBar(
+          const SnackBar(
+            content: Text('Comment posted'),
+            behavior: SnackBarBehavior.floating,
+            duration: Duration(seconds: 2),
+          ),
+        );
+      }
+    } on Exception catch (e) {
+      if (mounted) {
+        messenger.showSnackBar(
+          SnackBar(
+            content: Text('Failed to post comment: $e'),
+            behavior: SnackBarBehavior.floating,
+            backgroundColor: AppColors.primary,
+          ),
+        );
+      }
+      rethrow; // Let ReplyScreen know submission failed
+    }
+  }
+
+  /// Handle reply to a comment (nested reply)
+  Future<void> _handleCommentReply(
+    String content,
+    ThreadViewComment parentComment,
+  ) async {
+    final commentsProvider = context.read<CommentsProvider>();
+    final messenger = ScaffoldMessenger.of(context);
+
+    try {
+      await commentsProvider.createComment(
+        content: content,
+        parentComment: parentComment,
+      );
+
+      if (mounted) {
+        messenger.showSnackBar(
+          const SnackBar(
+            content: Text('Reply posted'),
+            behavior: SnackBarBehavior.floating,
+            duration: Duration(seconds: 2),
+          ),
+        );
+      }
+    } on Exception catch (e) {
+      if (mounted) {
+        messenger.showSnackBar(
+          SnackBar(
+            content: Text('Failed to post reply: $e'),
+            behavior: SnackBarBehavior.floating,
+            backgroundColor: AppColors.primary,
+          ),
+        );
+      }
+      rethrow; // Let ReplyScreen know submission failed
+    }
+  }
+
+  /// Open reply screen for replying to a comment
+  void _openReplyToComment(ThreadViewComment comment) {
+    // Check authentication
+    final authProvider = context.read<AuthProvider>();
+    if (!authProvider.isAuthenticated) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(
+          content: Text('Sign in to reply'),
+          behavior: SnackBarBehavior.floating,
+        ),
+      );
+      return;
+    }
+
+    // Navigate to reply screen with comment context
+    Navigator.of(context).push(
+      MaterialPageRoute<void>(
+        builder: (context) => ReplyScreen(
+          comment: comment,
+          onSubmit: (content) => _handleCommentReply(content, comment),
+        ),
       ),
     );
   }
···
                         comment: comment,
                         currentTimeNotifier:
                             commentsProvider.currentTimeNotifier,
+                        onCommentTap: _openReplyToComment,
                       );
                     },
                     childCount:
···
   const _CommentItem({
     required this.comment,
     required this.currentTimeNotifier,
+    this.onCommentTap,
   });
 
   final ThreadViewComment comment;
   final ValueNotifier<DateTime?> currentTimeNotifier;
+  final void Function(ThreadViewComment)? onCommentTap;
 
   @override
   Widget build(BuildContext context) {
···
           thread: comment,
           currentTime: currentTime,
           maxDepth: 6,
+          onCommentTap: onCommentTap,
         );
       },
     );

lib/widgets/comment_card.dart

···
 /// - Comment content (supports facets for links/mentions)
 /// - Heart vote button with optimistic updates via VoteProvider
 /// - Visual threading indicator based on nesting depth
+/// - Tap-to-reply functionality via [onTap] callback
 ///
 /// The [currentTime] parameter allows passing the current time for
 /// time-ago calculations, enabling periodic updates and testing.
···
     required this.comment,
     this.depth = 0,
     this.currentTime,
+    this.onTap,
     super.key,
   });
 
   final CommentView comment;
   final int depth;
   final DateTime? currentTime;
+
+  /// Callback when the comment is tapped (for reply functionality)
+  final VoidCallback? onTap;
 
   @override
   Widget build(BuildContext context) {
···
     // the stroke width)
     final borderLeftOffset = (threadingLineCount * 6.0) + 2.0;
 
-    return Container(
-      decoration: const BoxDecoration(color: AppColors.background),
-      child: Stack(
-        children: [
-          // Threading indicators - vertical lines showing nesting ancestry
-          Positioned.fill(
-            child: CustomPaint(
-              painter: _CommentDepthPainter(depth: threadingLineCount),
+    return InkWell(
+      onTap: onTap,
+      child: Container(
+        decoration: const BoxDecoration(color: AppColors.background),
+        child: Stack(
+          children: [
+            // Threading indicators - vertical lines showing nesting ancestry
+            Positioned.fill(
+              child: CustomPaint(
+                painter: _CommentDepthPainter(depth: threadingLineCount),
+              ),
+            ),
+            // Bottom border (starts after threading lines, not overlapping them)
+            Positioned(
+              left: borderLeftOffset,
+              right: 0,
+              bottom: 0,
+              child: Container(height: 1, color: AppColors.border),
             ),
-          ),
-          // Bottom border (starts after threading lines, not overlapping them)
-          Positioned(
-            left: borderLeftOffset,
-            right: 0,
-            bottom: 0,
-            child: Container(height: 1, color: AppColors.border),
-          ),
-          // Comment content with depth-based left padding
-          Padding(
-            padding: EdgeInsets.fromLTRB(leftPadding, 12, 16, 8),
-            child: Column(
-              crossAxisAlignment: CrossAxisAlignment.start,
-              children: [
-                // Author info row
-                Row(
-                  children: [
-                    // Author avatar
-                    _buildAuthorAvatar(comment.author),
-                    const SizedBox(width: 8),
-                    Expanded(
-                      child: Column(
-                        crossAxisAlignment: CrossAxisAlignment.start,
-                        children: [
-                          // Author handle
-                          Text(
-                            '@${comment.author.handle}',
-                            style: TextStyle(
-                              color: AppColors.textPrimary.withValues(
-                                alpha: 0.5,
+            // Comment content with depth-based left padding
+            Padding(
+              padding: EdgeInsets.fromLTRB(leftPadding, 12, 16, 8),
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: [
+                  // Author info row
+                  Row(
+                    children: [
+                      // Author avatar
+                      _buildAuthorAvatar(comment.author),
+                      const SizedBox(width: 8),
+                      Expanded(
+                        child: Column(
+                          crossAxisAlignment: CrossAxisAlignment.start,
+                          children: [
+                            // Author handle
+                            Text(
+                              '@${comment.author.handle}',
+                              style: TextStyle(
+                                color: AppColors.textPrimary.withValues(
+                                  alpha: 0.5,
+                                ),
+                                fontSize: 13,
+                                fontWeight: FontWeight.w500,
                               ),
-                              fontSize: 13,
-                              fontWeight: FontWeight.w500,
                             ),
-                          ),
-                        ],
+                          ],
+                        ),
                       ),
-                    ),
-                    // Time ago
-                    Text(
-                      DateTimeUtils.formatTimeAgo(
-                        comment.createdAt,
-                        currentTime: currentTime,
+                      // Time ago
+                      Text(
+                        DateTimeUtils.formatTimeAgo(
+                          comment.createdAt,
+                          currentTime: currentTime,
+                        ),
+                        style: TextStyle(
+                          color: AppColors.textPrimary.withValues(alpha: 0.5),
+                          fontSize: 12,
+                        ),
                       ),
-                      style: TextStyle(
-                        color: AppColors.textPrimary.withValues(alpha: 0.5),
-                        fontSize: 12,
-                      ),
-                    ),
+                    ],
+                  ),
+                  const SizedBox(height: 8),
+
+                  // Comment content
+                  if (comment.content.isNotEmpty) ...[
+                    _buildCommentContent(comment),
+                    const SizedBox(height: 8),
                   ],
-                ),
-                const SizedBox(height: 8),
 
-                // Comment content
-                if (comment.content.isNotEmpty) ...[
-                  _buildCommentContent(comment),
-                  const SizedBox(height: 8),
+                  // Action buttons (just vote for now)
+                  _buildActionButtons(context),
                 ],
-
-                // Action buttons (just vote for now)
-                _buildActionButtons(context),
-              ],
+              ),
             ),
-          ),
-        ],
+          ],
+        ),
       ),
     );
   }

lib/widgets/comment_thread.dart

···
 /// - Indents nested replies visually
 /// - Limits nesting depth to prevent excessive indentation
 /// - Shows "Load more replies" button when hasMore is true
+/// - Supports tap-to-reply via [onCommentTap] callback
 ///
 /// The [maxDepth] parameter controls how deeply nested comments can be
 /// before they're rendered at the same level to prevent UI overflow.
···
     this.maxDepth = 5,
     this.currentTime,
     this.onLoadMoreReplies,
+    this.onCommentTap,
     super.key,
   });
 
···
   final int maxDepth;
   final DateTime? currentTime;
   final VoidCallback? onLoadMoreReplies;
+
+  /// Callback when a comment is tapped (for reply functionality)
+  final void Function(ThreadViewComment)? onCommentTap;
 
   @override
   Widget build(BuildContext context) {
···
     return Column(
       crossAxisAlignment: CrossAxisAlignment.start,
       children: [
-        // Render the comment
+        // Render the comment with tap handler
         CommentCard(
           comment: thread.comment,
           depth: effectiveDepth,
           currentTime: currentTime,
+          onTap: onCommentTap != null ? () => onCommentTap!(thread) : null,
         ),
 
         // Render replies recursively
···
               maxDepth: maxDepth,
               currentTime: currentTime,
               onLoadMoreReplies: onLoadMoreReplies,
+              onCommentTap: onCommentTap,
             ),
           ),