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, 11 insertions, 0 deletions

diff --git a/web/css/player.css b/web/css/player.css
index 0f468f9..7a8b561 100644
--- a/web/css/player.css
+++ b/web/css/player.css
@@ -128,6 +128,17 @@
   max-height: 100%;
 }
 
+.player.crop-mode video {
+  object-fit: fill;
+}
+
+.crop-indicator {
+  font-size: 0.75rem;
+  color: var(--text-secondary);
+  padding: 0 0.25rem;
+  user-select: none;
+}
+
 /* Cover art for audio playback */
 .player .cover-art {
   width: 100%;