feat(models): add ViewerState to PostView for vote tracking

Add ViewerState class to represent the viewer's relationship with posts:
- vote: direction ("up", "down", or null)
- voteUri: AT-URI of the vote record
- saved: bookmark status
- tags: user-applied tags

This enables the feed to include viewer-specific state from the backend,
allowing proper initialization of vote UI state on feed refresh.

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

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

lib/models/comment.dart

···
 }
 
 class CommentViewerState {
-  CommentViewerState({this.vote});
+  CommentViewerState({this.vote, this.voteUri});
 
   factory CommentViewerState.fromJson(Map<String, dynamic> json) {
-    return CommentViewerState(vote: json['vote'] as String?);
+    return CommentViewerState(
+      vote: json['vote'] as String?,
+      voteUri: json['voteUri'] as String?,
+    );
   }
 
+  /// Vote direction: "up", "down", or null if not voted
   final String? vote;
+
+  /// AT-URI of the vote record (if backend provides it)
+  final String? voteUri;
 }

lib/models/post.dart

···
   final FeedReason? reason;
 }
 
+class ViewerState {
+  ViewerState({
+    this.vote,
+    this.voteUri,
+    this.saved = false,
+    this.savedUri,
+    this.tags,
+  });
+
+  factory ViewerState.fromJson(Map<String, dynamic> json) {
+    return ViewerState(
+      vote: json['vote'] as String?,
+      voteUri: json['voteUri'] as String?,
+      saved: json['saved'] as bool? ?? false,
+      savedUri: json['savedUri'] as String?,
+      tags: (json['tags'] as List<dynamic>?)?.cast<String>(),
+    );
+  }
+
+  /// Vote direction: "up", "down", or null if not voted
+  final String? vote;
+
+  /// AT-URI of the vote record
+  final String? voteUri;
+
+  /// Whether the post is saved/bookmarked
+  final bool saved;
+
+  /// AT-URI of the saved record
+  final String? savedUri;
+
+  /// User-applied tags
+  final List<String>? tags;
+}
+
 class PostView {
   PostView({
     required this.uri,
···
     required this.stats,
     this.embed,
     this.facets,
+    this.viewer,
   });
 
   factory PostView.fromJson(Map<String, dynamic> json) {
···
                   .map((f) => PostFacet.fromJson(f as Map<String, dynamic>))
                   .toList()
               : null,
+      viewer:
+          json['viewer'] != null
+              ? ViewerState.fromJson(json['viewer'] as Map<String, dynamic>)
+              : null,
     );
   }
   final String uri;
···
   final PostStats stats;
   final PostEmbed? embed;
   final List<PostFacet>? facets;
+  final ViewerState? viewer;
 }
 
 class AuthorView {