Add inline video preview to the file panel

[luckrnx09]

Closes #654.

Summary

Video files were not previewable: they fell through to the text-preview branch and were rejected above 256KB, and .webm was misclassified as audio. This adds a video preview path end to end.

Changes

• lib/file-types.ts: new VIDEO_EXT_TO_MIME (mp4/m4v/webm/mov/ogv) with getVideoMime() / isVideoPath(); webm moved out of AUDIO_EXT_TO_MIME (weba remains audio).
• app/api/files/[...path]/route.ts: read, meta, and download handle video MIME types via the existing Range-capable streamFile() — streaming and seeking work, no size cap (same as audio).
• components/FileViewer.tsx: new VideoViewer mirroring AudioViewer (live-watch badge, duration, size, download); dispatcher routes isVideoPath() to it.
• Tests: file-types.test.mjs covers video detection and the webm reclassification; FileViewer.state.test.mjs viewer-count/watch wiring updated for the fifth viewer.

Testing

• npm test: 846/846 pass; tsc --noEmit and eslint clean.
• Browser-verified against a production build with generated fixtures:
  • mp4 (H.264) and webm (VP9) play inline with controls and duration.
  • Range seek verified on a 45s mp4 (jump to 30s resumes playback; server returns 206).
  • Portrait (9:16) video is clamped to the panel height instead of overflowing.