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/controller/mastodon/settings.py

@@ -204,6 +204,7 @@ class accountSettingsController(globalSettingsController):
         all_buffers['blocked']=_("Blocked users")
         all_buffers['muted']=_("Muted users")
         all_buffers['notifications']=_("Notifications")
+        all_buffers['announcements']=_("Announcements")
         list_buffers = []
         hidden_buffers=[]
         all_buffers_keys = list(all_buffers.keys())