M1 — Tauri drop-zone + video player

11 chunks consolidated. Tauri 2 shell with vanilla HTML/CSS/JS frontend, MP4 drop-zone with convertFileSrc → <video> playback, view switching.

Full write-up in _docs/milestone-1-drop-zone-player.md.

The Leptos editor UI (Button / Card / DopeSheet) lives separately under ui-storybook and is not yet wired into the Tauri shell. Migration from vanilla JS to Leptos is queued as a future chunk.