Add embedded mpv player (render API + QOpenGLWidget)

New ytsage/gui/ytsage_gui_player.py:
- MpvRenderWidget hosts libmpv through MpvRenderContext in a
  QOpenGLWidget - works on native Wayland where wid-embedding cannot -
  with all mpv-thread callbacks marshalled to the GUI thread via queued
  signals only.
- PlayerPanel adds transport controls: play/pause, seek slider, time
  display, quality selector (caps ytdl-format height and reloads in
  place), speed, volume (persisted), subtitle toggle, fullscreen
  (reparent to top-level window), and Space/F/Escape keys.
- Playback resolves watch URLs through mpv's ytdl_hook pointed at the
  app-managed SHA256-verified yt-dlp binary (script-opts
  ytdl_hook-ytdl_path), inheriting cookies and proxy settings via
  ytdl-raw-options - stream freshness, DASH muxing and nsig handling
  stay in yt-dlp's hands.

New ytsage/core/ytsage_mpv.py probes libmpv availability; without it
the Watch UI shows a per-OS install hint and everything else works.
python-mpv added to dependencies (libmpv itself is a system package).
Config gains player.* and feed.* defaults.

Verified on Wayland: real YouTube video streams with position/duration
signals flowing and no thread-safety errors.

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

ytsage/languages/en.json

@@ -639,5 +639,14 @@
     "update_success": "✅ Deno has been successfully updated!",
     "update_failed": "❌ Update failed: {error}",
     "check_failed": "Failed to check for updates. Please check your internet connection."
+  },
+  "player": {
+    "quality_auto": "Auto",
+    "subtitles": "CC",
+    "unavailable": "Video player unavailable",
+    "now_playing": "Now Playing",
+    "queue": "Queue",
+    "play": "Play",
+    "pause": "Pause"
   }
 }