fix(hotkeys): name the full combo when its key-up reaches us

The message could only cite the modifiers ("after Ctrl+Shift") because
Windows consumes the KEYDOWN of a combo it routed to another app. Its KEYUP
often still arrives though — when it does, report the exact combo the user
tried: "Ctrl+Shift+T is already used by another app".

Falls back to the modifiers-only wording when the key-up is swallowed too.
Can't misfire: an accepted combo ends the capture on keydown, so a non-modifier
key-up during capture means that key was swallowed.

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

js/app.js

@@ -7812,6 +7812,21 @@ async function init() {
             // the user staring at "Press combo…".
             function onKeyUp(e) {
                 if (!heldMods) return;
+                // Windows consumes the KEYDOWN of a combo it routed elsewhere,
+                // but the KEYUP of the final key often still reaches us. When it
+                // does we can name the exact combo the user tried; otherwise we
+                // fall back to the modifiers below. This can only fire on a
+                // swallowed key — an accepted combo ends the capture on keydown.
+                const k = e.key || '';
+                const isMod = /^(Control|Shift|Alt|Meta|OS)$/.test(k);
+                if (!isMod &&
+                    ((k.length === 1 && /[a-z0-9]/i.test(k)) || /^F([1-9]|1[0-2])$/i.test(k))) {
+                    const combo = heldMods + '+' + k.toUpperCase();
+                    finish(true);
+                    toast(combo + ' is already used by another app — the keystroke ' +
+                          'never reaches us. Pick a different combo.', 'warning');
+                    return;
+                }
                 if (e.ctrlKey || e.shiftKey || e.altKey || e.metaKey) return;
                 const mods = heldMods;
                 finish(true);