fix(hotkeys): capture times out instead of waiting forever

A combo another app already registered is consumed by Windows and delivered
to that app — our keydown listener never fires. The capture therefore sat on
"Press combo…" indefinitely with no feedback, which is exactly what a user
sees when they try to re-assign a taken combo.

Worse, while armed the capture preventDefaults every keystroke, so an
abandoned capture froze the app's keyboard until Esc.

10 s timeout that restores the button and explains the likely cause: the
combo never reached us because another app owns it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/app.js

@@ -7708,7 +7708,20 @@ async function init() {
             const original = btn.textContent;
             btn.textContent = 'Press combo… (Esc to cancel)';
 
+            // A combo already owned by another app via RegisterHotKey is
+            // swallowed by Windows — it is delivered to that app, never to us,
+            // so the capture would wait forever with no explanation. It also
+            // preventDefaults every keystroke while armed, so an abandoned
+            // capture freezes the app's keyboard. Time out and say why.
+            const timer = setTimeout(() => {
+                finish(true);
+                toast('No combo captured. If you did press one, another app ' +
+                      'already owns it — Windows routes it there and we never ' +
+                      'see it. Try a different combo.', 'warning');
+            }, 10000);
+
             function finish(restore) {
+                clearTimeout(timer);
                 btn.dataset.capturing = '';
                 btn.classList.remove('is-capturing');
                 document.removeEventListener('keydown', onKey, true);