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.

1 files changed, 1 insertions, 0 deletions

diff --git a/web/js/keyboard.js b/web/js/keyboard.js
index 951e573..dc3eb4e 100644
--- a/web/js/keyboard.js
+++ b/web/js/keyboard.js
@@ -214,6 +214,7 @@ export function initKeyboard(handlers) {
         break;
       case 'C': handlers.toggleMinimize?.(e); break;
       case 'f': handlers.fullscreen?.(e); break;
+      case 'c': handlers.toggleCrop?.(e); break;
       case 'Escape': handlers.escape?.(e); break;
       case 'Backspace': handlers.backspace?.(e); break;
       case 'r': handlers.shuffle?.(e); break;