feat(media): add immersive full-screen video playback

[veryCrunchy]

User problem

Native video playback uses the media viewer canvas but does not provide an explicit immersive full-screen mode. Users should be able to watch a video without persistent application chrome and without downloading it or opening it in another application.

Acceptance criteria

• Add an obvious full-screen control to native video playback.
• Hide application chrome and, where Android permits, system bars during immersive playback.
• Reveal playback controls temporarily through an accessible user interaction.
• Android Back exits full-screen before closing the media viewer.
• Preserve playback position, pause state, volume state, and selected media when entering or leaving full-screen.
• Rotation and window resizing do not restart the stream or lose playback position.
• Landscape uses the complete available display without unintended stretching or cropping.
• Controls remain keyboard-, touch-, and screen-reader accessible.
• Restore system-bar and orientation state when playback closes, fails, or the app moves to the background.
• Add deterministic phone, tablet, portrait, and landscape coverage.

Relationships

• Parent: [PHOTO-002] Video playback #83
• Related landscape behavior: [UX-004] Landscape interaction and layout #151
• Related viewer-canvas work: fix(media): use the full landscape canvas in the photo viewer #194