Make screen capture work in Firefox

Firefox has no tab capture, so preferCurrentTab is silently dropped and the
operator can only share a window or a screen. Neither maps onto the viewport
the way the crop assumes, so screenshots came out offset by the browser chrome
— and every failure was swallowed by a bare catch, leaving no way to tell why.

Check the shared surface and only crop when the viewport covers it: exact for
Chrome's tab capture, true for a screen while the page is fullscreen. Test that
by dimensions rather than document.fullscreenElement, which F11 leaves null.
Start now requests fullscreen, after getDisplayMedia so the click's transient
activation goes to the fragile call. Reasons surface in a corner notice and the
console, and the check reruns per grab so leaving fullscreen says so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H9v2Xoa6eoeGYpK6gp95k9

public/player/player.css

@@ -318,3 +318,18 @@ body {
   letter-spacing: 0.08em;
   text-transform: uppercase;
 }
+
+/* Bottom left, opposite #status, so the two never overlap. */
+#notice {
+  position: fixed;
+  bottom: 12px;
+  left: 16px;
+  z-index: 20;
+  max-width: min(46ch, 60vw);
+  padding: 8px 12px;
+  border-radius: 6px;
+  background: rgb(0 0 0 / 0.55);
+  color: var(--accent);
+  font-size: 0.8rem;
+  line-height: 1.35;
+}