feat(video): real decode path for .avi / .mpeg / .mj2 (split from #157)

[alicup29]

Split out from #157. The cleanup half of #157 is complete and shipped — sleap-io.js#165 (released in 0.5.7) removes .avi/.mpeg/.mpg from MEDIABUNNY_EXTENSIONS and raises a clean UnsupportedVideoFormatError, and app-side src/lib/resolveVideos.ts gates those extensions out with a toast (no crash) via #156. This issue tracks the remaining feature work only.

Problem

sleap-app / sleap-io.js have no decode path for .avi, .mpeg/.mpg (MPEG program stream), or .mj2. Opening one today surfaces "unsupported format — transcode to MP4 first." This is a genuine PyQt-SLEAP parity gap (PyQt decodes these via its Python video backends).

Options to scope

• Desktop ffmpeg sidecar (Tauri): bundle an ffmpeg binary and decode natively. Fast, but desktop-only, and adds a binary dependency + per-platform packaging.
• WASM decode path (ffmpeg.wasm / libav.js): works in the browser too, at software-decode speed. See the Linux-MediaVideo WASM-decode investigation (docs/plans/2026-07-27-linux-mediavideo-wasm-decode-design.md) for the libav.js + MediaBunny registerDecoder pattern this could reuse.
• Upstream MediaBunny adds AVI / MPEG-PS demux (Add AVI container format support Vanilagy/mediabunny#155 — currently noted as "not a goal").
• Keep transcode-on-import as the sanctioned workflow and just document it clearly (lowest effort).

Acceptance

• Decide the approach above.
• Open (or scrub keypoints on) an .avi and an .mpeg without transcoding — OR document the transcode workflow as the intended path.

Once this is filed, #157 can be closed (its cleanup scope is done).