Add comprehensive direct messaging and bookmarks system

- Add dedicated Messages timeline with conversation threading
- Implement bookmarks timeline for saved posts
- Support both standard ActivityPub conversations and Pleroma chats
- Auto-mark conversations as read when selected
- Full pagination support for messages and bookmarks
- Reorder timeline tabs for better UX (Messages now second tab)
- Remove verbose prefixes for cleaner accessibility
- Add conversation management API endpoints
- Maintain accessibility-first design with no text truncation

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

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

src/main_window.py

@@ -69,9 +69,11 @@ class MainWindow(QMainWindow):
         self.timeline_tabs = QTabWidget()
         self.timeline_tabs.setAccessibleName("Timeline Selection")
         self.timeline_tabs.addTab(QWidget(), "Home")
+        self.timeline_tabs.addTab(QWidget(), "Messages")
         self.timeline_tabs.addTab(QWidget(), "Mentions")
         self.timeline_tabs.addTab(QWidget(), "Local")
         self.timeline_tabs.addTab(QWidget(), "Federated")
+        self.timeline_tabs.addTab(QWidget(), "Bookmarks")
         self.timeline_tabs.addTab(QWidget(), "Followers")
         self.timeline_tabs.addTab(QWidget(), "Following")
         self.timeline_tabs.currentChanged.connect(self.on_timeline_tab_changed)
@@ -447,8 +449,8 @@ class MainWindow(QMainWindow):
         
     def switch_timeline(self, index):
         """Switch to timeline by index with loading feedback"""
-        timeline_names = ["Home", "Mentions", "Local", "Federated", "Followers", "Following"]
-        timeline_types = ["home", "notifications", "local", "federated", "followers", "following"]
+        timeline_names = ["Home", "Messages", "Mentions", "Local", "Federated", "Bookmarks", "Followers", "Following"]
+        timeline_types = ["home", "conversations", "notifications", "local", "federated", "bookmarks", "followers", "following"]
         
         if 0 <= index < len(timeline_names):
             timeline_name = timeline_names[index]