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/gui/ytsage_gui_browse.py

@@ -0,0 +1,23 @@
+"""
+Browse tab - channel and playlist browsing (placeholder, implemented next)
+"""
+
+from typing import Optional
+
+from PySide6.QtCore import Qt
+from PySide6.QtWidgets import QLabel, QVBoxLayout, QWidget
+
+from ..utils.ytsage_localization import _
+
+
+class BrowsePage(QWidget):
+    def __init__(self, router, parent: Optional[QWidget] = None) -> None:
+        super().__init__(parent)
+        self._router = router
+        layout = QVBoxLayout(self)
+        label = QLabel(_("browse.coming_soon"))
+        label.setAlignment(Qt.AlignmentFlag.AlignCenter)
+        layout.addWidget(label)
+
+    def open_url(self, url: str) -> None:
+        pass