Add favorites timeline tab

- Add favorites timeline as third tab (position 2) after Messages
- Implement get_favorites() API method using /api/v1/favourites endpoint
- Add timeline handling for favorites in load_posts() and load_more_posts()
- Include empty state message for when no favorites exist
- Update all timeline conditionals to include favorites support

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

Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -723,3 +723,5 @@ content_text.setTextInteractionFlags(Qt.TextSelectableByKeyboard | Qt.TextSelect
 
 This document serves as the comprehensive development guide for Bifrost, ensuring all accessibility, functionality, and architectural decisions are preserved and can be 
 referenced throughout development.
+
+- When adding or adjusting features be extremely thorough. Make sure every aspect of the code having to do with the feature or fix is completely updated.