fix(profile): add sign out option when profile fails to load

Prevents users from being trapped in the app when their atProto profile
is misconfigured or not found. Adds optional secondary action support
to FullScreenError widget.

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

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

lib/screens/home/profile_screen.dart

···
     // Show error state
     if (profileProvider.profileError != null &&
         profileProvider.profile == null) {
+      // Only show sign out option for own profile (no actor param)
+      // This prevents users from being trapped with a misconfigured profile
+      final isOwnProfile = widget.actor == null;
+
       return Scaffold(
         backgroundColor: AppColors.background,
         appBar: _buildAppBar(context, null),
···
           title: 'Failed to load profile',
           message: profileProvider.profileError!,
           onRetry: () => profileProvider.retryProfile(),
+          secondaryActionLabel: isOwnProfile ? 'Sign Out' : null,
+          onSecondaryAction: isOwnProfile ? _handleSignOut : null,
+          secondaryActionDestructive: true,
         ),
       );
     }

lib/widgets/loading_error_states.dart

···
   }
 }
 
-/// Full-screen error state with retry button
+/// Full-screen error state with retry button and optional secondary action
 class FullScreenError extends StatelessWidget {
   const FullScreenError({
     required this.message,
     required this.onRetry,
     this.title = 'Failed to load',
+    this.secondaryActionLabel,
+    this.onSecondaryAction,
+    this.secondaryActionDestructive = false,
     super.key,
   });
 
   final String title;
   final String message;
   final VoidCallback onRetry;
+
+  /// Optional secondary action button label (e.g., "Sign Out")
+  final String? secondaryActionLabel;
+
+  /// Optional secondary action callback
+  final VoidCallback? onSecondaryAction;
+
+  /// Whether the secondary action is destructive (shows in red)
+  final bool secondaryActionDestructive;
 
   @override
   Widget build(BuildContext context) {
···
               ),
               child: const Text('Retry'),
             ),
+            if (secondaryActionLabel != null && onSecondaryAction != null) ...[
+              const SizedBox(height: 12),
+              TextButton(
+                onPressed: onSecondaryAction,
+                style: TextButton.styleFrom(
+                  foregroundColor: secondaryActionDestructive
+                      ? Colors.red.shade400
+                      : AppColors.textSecondary,
+                ),
+                child: Text(secondaryActionLabel!),
+              ),
+            ],
           ],
         ),
       ),