Restructure main window into a watch-first tab shell

The single-page downloader layout becomes the Downloads tab of a
SmoothTabWidget with Watch / Search / Feed / Browse / Downloads pages.
The init_ui edit is deliberately small (the old central widget is now
self.download_page); all new behavior lives in new modules:

- ytsage_gui_router.py: AppRouter signal hub (playVideo, queueVideo,
  downloadVideo, openChannel, openPlaylist). A card's Download button
  deep-links into the Downloads tab with the URL prefilled and analysis
  started automatically.
- ytsage_gui_cards.py: VideoCard (thumbnail with disk cache under
  APP_THUMBNAILS_DIR, title/channel/duration, Play/Queue/Download
  actions, double-click to play) and VideoCardGrid (responsive grid,
  Load more pagination).
- ytsage_gui_watch.py: WatchPage hosting the mpv PlayerPanel and a
  drag-reorderable play queue with auto-advance on end of file.
- ytsage_gui_search.py: SearchPage running YtdlpClient.search off the
  GUI thread with Load more pagination.
- Browse and Feed pages are placeholders, implemented next.

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

ytsage/languages/en.json

@@ -648,5 +648,35 @@
     "queue": "Queue",
     "play": "Play",
     "pause": "Pause"
+  },
+  "cards": {
+    "play": "▶ Play",
+    "queue": "+ Queue",
+    "download": "⬇",
+    "load_more": "Load more",
+    "empty": "Nothing here yet"
+  },
+  "main_tabs": {
+    "watch": "Watch",
+    "search": "Search",
+    "feed": "Feed",
+    "browse": "Browse",
+    "downloads": "Downloads"
+  },
+  "search": {
+    "placeholder": "Search YouTube...",
+    "button": "Search",
+    "searching": "Searching...",
+    "results_count": "{count} results",
+    "failed": "Search failed: {error}"
+  },
+  "watch": {
+    "clear_queue": "Clear queue"
+  },
+  "browse": {
+    "coming_soon": "Channel & playlist browsing coming soon"
+  },
+  "feed": {
+    "coming_soon": "Subscriptions feed coming soon"
   }
 }