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/utils/ytsage_config_manager.py

@@ -109,6 +109,17 @@ class ConfigManager:
             # the app-managed, SHA256-verified binary is absent
             "allow_system_ytdlp": False,
         },
+        "player": {
+            "default_quality": 1080,  # max stream height; None = auto/best
+            "volume": 100,
+            "resume": "auto",  # auto | off
+            "source_mode": "ytdl",  # ytdl (mpv ytdl_hook) | direct (raw stream URLs)
+        },
+        "feed": {
+            "mode": "local",  # local (per-channel aggregation) | account (cookies)
+            "per_channel_items": 15,
+            "auto_refresh_minutes": 0,  # 0 = manual refresh only
+        },
     }
 
     @classmethod