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_search.py

@@ -0,0 +1,91 @@
+"""
+Search tab - in-app YouTube search via yt-dlp (ytsearchN:)
+"""
+
+from typing import Any, Dict, List, Optional
+
+from PySide6.QtWidgets import QHBoxLayout, QLabel, QLineEdit, QPushButton, QVBoxLayout, QWidget
+
+from .ytsage_gui_cards import VideoCardGrid
+from ..core.ytsage_client import YtdlpWorker
+from ..utils.ytsage_localization import _
+from ..utils.ytsage_logger import logger
+
+PAGE_SIZE = 24
+
+
+class SearchPage(QWidget):
+    def __init__(self, router, parent: Optional[QWidget] = None) -> None:
+        super().__init__(parent)
+        self._router = router
+        self._worker: Optional[YtdlpWorker] = None
+        self._query = ""
+        self._offset = 0
+
+        layout = QVBoxLayout(self)
+        layout.setContentsMargins(8, 8, 8, 8)
+
+        bar = QHBoxLayout()
+        self.query_input = QLineEdit()
+        self.query_input.setPlaceholderText(_("search.placeholder"))
+        self.query_input.returnPressed.connect(self.start_search)
+        self.query_input.setMinimumHeight(38)
+        bar.addWidget(self.query_input, stretch=1)
+
+        self.search_btn = QPushButton(_("search.button"))
+        self.search_btn.clicked.connect(self.start_search)
+        self.search_btn.setMinimumHeight(38)
+        bar.addWidget(self.search_btn)
+        layout.addLayout(bar)
+
+        self.status_label = QLabel("")
+        self.status_label.setStyleSheet("color: #9aa0a6; padding: 2px;")
+        layout.addWidget(self.status_label)
+
+        self.grid = VideoCardGrid(router, self)
+        self.grid.loadMoreRequested.connect(self.load_more)
+        layout.addWidget(self.grid, stretch=1)
+
+    # ---------------------------------------------------------------- search
+
+    def start_search(self) -> None:
+        query = self.query_input.text().strip()
+        if not query or self._worker is not None:
+            return
+        self._query = query
+        self._offset = 0
+        self.grid.clear()
+        self._fetch(append=False)
+
+    def load_more(self) -> None:
+        if self._worker is None and self._query:
+            self._offset += PAGE_SIZE
+            self._fetch(append=True)
+
+    def _fetch(self, append: bool) -> None:
+        self.status_label.setText(_("search.searching"))
+        self.search_btn.setEnabled(False)
+        query, offset = self._query, self._offset
+        self._worker = YtdlpWorker(lambda c: c.search(query, n=PAGE_SIZE, offset=offset))
+        self._worker.result.connect(lambda entries: self._on_results(entries, append))
+        self._worker.error.connect(self._on_error)
+        self._worker.finished.connect(self._on_finished)
+        self._worker.start()
+
+    def _on_results(self, entries: List[Dict[str, Any]], append: bool) -> None:
+        show_more = len(entries) >= PAGE_SIZE
+        if append:
+            self.grid.append_entries(entries, show_load_more=show_more)
+        else:
+            self.grid.set_entries(entries, show_load_more=show_more)
+        self.status_label.setText(_("search.results_count", count=self.grid.card_count()))
+
+    def _on_error(self, message: str) -> None:
+        logger.error(f"Search failed: {message}")
+        self.status_label.setText(_("search.failed", error=message[:200]))
+
+    def _on_finished(self) -> None:
+        self.search_btn.setEnabled(True)
+        if self._worker is not None:
+            self._worker.deleteLater()
+            self._worker = None