feat: add a controller-first full-screen video player

[nextestudios]

Summary

Implements the basic in-app video preview (#61) and the controller-first full-screen player (#170) in one PR, since #170 builds directly on #61.

• Open: South on MP4/M4V/MOV/WMV/AVI/MKV/WebM/3GP from the list or grid (or North → Assistir aqui). The existing Open/Open with items stay.
• Full screen: a new VideoPlayerView layer covers the whole window under the modal layer, so menus/dialogs open on top without recreating the video element; the window switches to full screen while a video is open and restores afterwards (a user F11 is left alone).
• Overlay: title, elapsed/remaining time, progress bar, volume/mute, subtitle and audio-track state, and the active controller's prompts (feat: add dynamic controller button prompts to action bar #34). Shows on any input, fades after 3 s while playing, stays while paused; prompts only when the player is the top modal.
• Controls (semantic actions only): South play/pause (restart at end); Left/Right ±10 s, LB/RB ±1 min, LT/RT ±5% (timeline) accumulate a previewed target shown large and as a mark on the bar, committed after 700 ms (South commits now, Back cancels), bounded to [0, duration]; Up/Down volume; North options menu (subtitles off/embedded/sidecar .srt/.vtt, audio track when >1, mute, start over, forget position) with unavailable entries disabled and explained; Back hides the overlay first, then closes and restores folder + focused file.
• Resume: leaving between 30 s and duration−30 s stores the position under a SHA-256 of path+size+mtime (never the name) in playback.json (max 500). Reopening the same file asks Continuar de m:ss / Começar do início / back; another file with the same name or an edited file does not resume. Ending the video or "Esquecer onde parei" forgets it; Configurações → Apagar onde os vídeos pararam clears all.
• Errors: unsupported codecs / no audio device show a clear message on the overlay; only Back is offered and it returns to the list. Playback stops and the file is released on close.
• Windows codecs only (documented in GUIDE both languages: which formats may need Store extensions). Security notes in docs/security-model.md, privacy note in docs/PRIVACY.md. Render harness: mv-video-player with a generated uncompressed AVI.

Tested in CI

• VideoPlayerJourneyTests (fake player/store/clock): overlay fade and reveal, previewed seek accumulation/commit/bounds/cancel, Back hides overlay before closing, focus restored; resume offered only for the same file, resume applied when seekable, subtitles selectable and missing audio tracks explained, ended video forgets the position; unsupported codec message with Back only.

Manual checks (docs/TESTING.md, "Vídeo (#61, #170)")

• Real playback, full-screen switch, multi-track MKV/embedded and sidecar subtitles, resume across launches, missing HEVC/VP9 codecs, TV/handheld legibility, real controllers per family. The GitHub runner has no audio device, so real playback can't be proven in CI (see the mv-video-player capture).

Closes #61
Closes #170

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