feat(mastodon): Add support for server announcements

Implemented a new 'Announcements' buffer to view instance-wide news. Features include:

- New buffer and UI panel for announcements.

- Support for templates and rendering of announcement content.

- 'Dismiss' functionality (mapped to Enter/Return) to mark announcements as read.

- Integrated into account settings for buffer management.

src/sessions/mastodon/compose.py

@@ -84,4 +84,10 @@ def compose_notification(notification, db, settings, relative_times, show_screen
     filtered = utils.evaluate_filters(post=notification, current_context="notifications")
     if filtered != None:
         text = _("hidden by filter {}").format(filtered)
-    return [user, text, ts]
+    return [user, text, ts]
+
+def compose_announcement(announcement, db, settings, relative_times, show_screen_names, safe=False):
+    # Use the default template or a configured one if available
+    template = settings.get("templates", {}).get("announcement", templates.announcement_default_template)
+    text = templates.render_announcement(announcement, template, settings, relative_times, db["utc_offset"])
+    return [text]