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_watch.py

@@ -0,0 +1,125 @@
+"""
+Watch tab - player plus play queue
+==================================
+
+Hosts the embedded mpv PlayerPanel (or the libmpv-missing hint) and a simple
+play queue. Entries arrive via AppRouter.playVideo / queueVideo.
+"""
+
+from typing import Any, Dict, List, Optional
+
+from PySide6.QtCore import Qt
+from PySide6.QtWidgets import (
+    QHBoxLayout,
+    QLabel,
+    QListWidget,
+    QListWidgetItem,
+    QPushButton,
+    QSplitter,
+    QVBoxLayout,
+    QWidget,
+)
+
+from .ytsage_gui_player import PlayerPanel, create_player_panel
+from ..utils.ytsage_localization import _
+from ..utils.ytsage_logger import logger
+
+
+class WatchPage(QWidget):
+    def __init__(self, router, parent: Optional[QWidget] = None) -> None:
+        super().__init__(parent)
+        self._router = router
+        self._queue: List[Dict[str, Any]] = []
+
+        layout = QVBoxLayout(self)
+        layout.setContentsMargins(8, 8, 8, 8)
+
+        splitter = QSplitter(Qt.Orientation.Horizontal, self)
+        layout.addWidget(splitter)
+
+        self.player = create_player_panel(self)
+        splitter.addWidget(self.player)
+
+        queue_panel = QWidget(self)
+        queue_layout = QVBoxLayout(queue_panel)
+        queue_layout.setContentsMargins(4, 0, 0, 0)
+
+        queue_header = QHBoxLayout()
+        queue_label = QLabel(_("player.queue"))
+        queue_label.setStyleSheet("font-weight: bold;")
+        queue_header.addWidget(queue_label)
+        queue_header.addStretch()
+        self.clear_queue_btn = QPushButton("✕")
+        self.clear_queue_btn.setFixedWidth(28)
+        self.clear_queue_btn.setToolTip(_("watch.clear_queue"))
+        self.clear_queue_btn.clicked.connect(self.clear_queue)
+        queue_header.addWidget(self.clear_queue_btn)
+        queue_layout.addLayout(queue_header)
+
+        self.queue_list = QListWidget()
+        self.queue_list.setDragDropMode(QListWidget.DragDropMode.InternalMove)
+        self.queue_list.itemDoubleClicked.connect(self._on_queue_item_activated)
+        self.queue_list.model().rowsMoved.connect(self._on_rows_moved)
+        queue_layout.addWidget(self.queue_list)
+
+        splitter.addWidget(queue_panel)
+        splitter.setStretchFactor(0, 4)
+        splitter.setStretchFactor(1, 1)
+        splitter.setSizes([900, 240])
+
+        if isinstance(self.player, PlayerPanel):
+            self.player.playbackEnded.connect(self._on_playback_ended)
+
+    # ------------------------------------------------------------ public API
+
+    def play_entry(self, entry: Dict[str, Any], resume_pos: float = 0.0) -> None:
+        if isinstance(self.player, PlayerPanel):
+            self.player.play(entry, resume_pos=resume_pos)
+        else:
+            logger.warning("Play requested but libmpv is unavailable")
+
+    def enqueue(self, entry: Dict[str, Any]) -> None:
+        self._queue.append(dict(entry))
+        item = QListWidgetItem(entry.get("title") or entry.get("url") or "?")
+        item.setData(Qt.ItemDataRole.UserRole, dict(entry))
+        self.queue_list.addItem(item)
+        # Start playing right away when nothing is on and this is the first item
+        if isinstance(self.player, PlayerPanel) and not self.player.current_entry() and len(self._queue) == 1:
+            self._play_next_from_queue()
+
+    def clear_queue(self) -> None:
+        self._queue.clear()
+        self.queue_list.clear()
+
+    def queue_entries(self) -> List[Dict[str, Any]]:
+        return [self.queue_list.item(i).data(Qt.ItemDataRole.UserRole) for i in range(self.queue_list.count())]
+
+    # --------------------------------------------------------------- internal
+
+    def _play_next_from_queue(self) -> None:
+        if self.queue_list.count() == 0:
+            return
+        item = self.queue_list.takeItem(0)
+        entry = item.data(Qt.ItemDataRole.UserRole)
+        if entry in self._queue:
+            self._queue.remove(entry)
+        self.play_entry(entry)
+
+    def _on_playback_ended(self, reason: str) -> None:
+        if reason in ("eof", "") and self.queue_list.count() > 0:
+            self._play_next_from_queue()
+
+    def _on_queue_item_activated(self, item: QListWidgetItem) -> None:
+        entry = item.data(Qt.ItemDataRole.UserRole)
+        row = self.queue_list.row(item)
+        self.queue_list.takeItem(row)
+        if entry in self._queue:
+            self._queue.remove(entry)
+        self.play_entry(entry)
+
+    def _on_rows_moved(self, *args) -> None:
+        self._queue = self.queue_entries()
+
+    def shutdown(self) -> None:
+        if isinstance(self.player, PlayerPanel):
+            self.player.shutdown()