Add local subscriptions and the Feed tab

New ytsage/utils/ytsage_library_manager.py: sagetube_library.db
(separate from the upstream download-history DB) with WAL from day one,
holding subscriptions, cached feed_items, watch_history with resume
positions, and the persisted play_queue. Watch history and queue tables
are wired up by the next commit.

FeedPage:
- Local mode: FeedRefreshWorker refreshes each subscribed channel
  sequentially (feed.per_channel_items, default 15) and the grid fills
  incrementally per channel; results are cached so the feed is
  populated instantly on startup.
- Account mode: fetches youtube.com/feed/subscriptions with the user's
  browser cookies - the real logged-in feed; the option is enabled only
  while cookies are active and errors surface as a status banner.
- Sidebar lists subscriptions (double-click opens the channel in
  Browse; context menu unsubscribes). Browse's Subscribe button now
  toggles subscription state through the Feed page.

Verified live: subscribe -> refresh -> 15 videos cached in SQLite and
rendered as cards.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ytsage/gui/ytsage_gui_main.py

@@ -659,6 +659,7 @@ class YTSageApp(QMainWindow, FormatTableMixin, VideoInfoMixin, AnalysisMixin):
         self.router.downloadVideo.connect(self._route_download_video)
         self.router.openChannel.connect(self._route_open_channel)
         self.router.openPlaylist.connect(self._route_open_playlist)
+        self.browse_page.subscribeRequested.connect(self.feed_page.subscribe_channel)
 
     def _tab_index_of(self, page) -> int:
         for i in range(self.main_tabs.stack.count()):