feat(windows): mpv draws Windows video into the app window

DR-237's video half. mpv now renders Windows video the way
tauri-plugin-libmpv does on Windows: MpvBackend is handed the main
window's HWND and sets it as `wid` before mpv initialises, so mpv draws
as a child of the app window beneath the transparent WebView2, with
vo=gpu-next,gpu. osc, default bindings, VO keyboard and cursor handling
are off, so the Svelte controls drawn over the picture are the only ones.

- video_output() decides per platform (UT-274): Window(hwnd) on Windows,
  RenderApi on Linux, Off without native video. Windows with no handle
  draws nothing rather than letting mpv open a top-level window.
- native_video::enabled() is true on Windows as well, so the frontend
  takes the native path there: NativePlayerAdapter, and the page clears
  its background while video is on screen.
- enableNativeVideoCompositing() no longer logs a missing Android bridge
  as an error on the desktop, where there is no bridge to have.

Verified: every option, wid included, is accepted by the real libmpv on
Linux and by the shipped Windows DLL under wine; the Windows unit suite
passes under wine (949). Not yet seen on real Windows hardware.

docs/requirements.md

@@ -869,6 +869,7 @@ Internal architecture, components, and application logic.
 | UT-271 | Native video is on for Linux whatever `JELLYTAU_NATIVE_VIDEO` says, including unset and explicit "off" values, and off where mpv is not the video renderer | DR-235 | Done |
 | UT-272 | No platform reports a webview video fallback, Linux reports native video, and the player status on Linux never sends video to the `<video>` element | DR-235 | Done |
 | UT-273 | `player_play_item`, `get_player_status` and `player_get_capabilities` answer "does a native renderer draw video here" from one function, so the backend is loaded with video exactly where the frontend is told not to use a `<video>` element — on Windows, where mpv now plays audio, the film's soundtrack is not decoded twice | DR-237 | Done |
+| UT-274 | mpv's video output is decided per platform: Windows renders into the app window's HWND (`wid`, set before initialisation) with `vo=gpu-next,gpu` and mpv's own controller, bindings and cursor handling off; Windows with no handle draws nothing rather than opening a window of its own; Linux uses the render API; no native video means `video=no`. Every option is accepted by the real libmpv, on Linux and by the shipped Windows DLL | DR-237 | Done |
 ### Integration Tests
 
 | Test ID | Test Description | Traces To | Status |