Add fullscreen crop mode toggle (hotkey c)

- In fullscreen, press 'c' to toggle object-fit between contain (default)
  and fill (stretches to fill, no aspect ratio preservation).
- Adds .player.crop-mode video { object-fit: fill; } in player.css.
- Adds toggleCrop() in player.js that toggles a crop-mode class and a
  small UI indicator (✂️) in the controls, visible only in fullscreen.
- Resets crop mode back to contain when exiting fullscreen (via Escape,
  minimize, or fullscreenchange event).
- Wires the 'c' hotkey in keyboard.js and registers toggleCrop in app.js.
- Adds crop-indicator to index.html and detach.html controls, and
  documents the shortcut in the help modal.

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