feat: add in-app video preview #61

Summary

Basic video preview with controller controls (advanced playback stays out of scope).

Motivation

Check a video without leaving ControlFS.

Requirements

• Play/pause, seek, full screen; codecs limited to what Windows supports.

Controller UX

South/A play/pause; Left/Right seek; East/B closes.

Technical Notes

WinUI MediaPlayerElement.

Acceptance Criteria

• Common MP4 files play; unsupported codecs show a clear message.

Testing

Unit tests for new branches and for every bug reproduced before a fix (no trivial variations, see AGENTS.md).

Dependencies

• None

Priority: low · Milestone: 5 · Preview & Editing