feat(viewer): WMV cutscene player with scrub, keyboard + audio

Add an in-explorer video player for the dat/movie/*.wmv cutscenes.

Decode via the ffmpeg/ffprobe CLI (no libav linking): video streams from an
`ffmpeg -f rawvideo -pix_fmt rgba` pipe read into fixed w*h*4 frame chunks over
a bounded channel (paused/behind => ffmpeg back-pressures on its pipe); one
reused Image is overwritten in place each tick. Audio is best-effort: the track
is pre-decoded to a temp stereo WAV and played through a rodio Sink (volume /
play / pause / seek-via-skip_duration for free). Native-only under
cfg(not(wasm32)); VideoPreview registers unconditionally so the UI compiles for
wasm. Mirrors the .pak load pipeline (FileSelected -> bg thread ->
IsoLoaderMsg::VideoLoaded -> apply_loaded_video -> advance_video_playback), with
a single free_video teardown wired into every other viewer's reset.

Controls:
- play/pause button, click-on-frame toggle, Space / arrow (+-10s) shortcuts
- seekable timeline with mm:ss labels; volume slider (greyed when no audio)
- live scrubbing: a coalescing one-shot `ffmpeg -ss -frames:v 1` grabber always
  jumps to the knob's latest position, so dragging shows the frame under the
  knob in real time; that grabbed frame also bridges the gap after a commit-seek
  so there's no black flash while the streaming decoder catches up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/sylpheed-viewer/Cargo.toml

@@ -50,3 +50,5 @@ dev = ["bevy/dynamic_linking"]
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 tracing-subscriber = { workspace = true }
 rfd                = { workspace = true }
+# Audio for the WMV video player (pulls cpal → alsa on Linux).
+rodio              = { workspace = true }