revert: remove redundant QuickSwitcher (Ctrl+K already does this better)

The existing SearchModalRenderer (Ctrl+K) is already a polished room/DM
switcher with avatars, unread badges, fuzzy search, and keyboard nav.
Our QuickSwitcher was an inferior duplicate. Removing it entirely:
- Delete QuickSwitcher.tsx
- Remove QuickSwitcherFeature from ClientNonUIFeatures
- Remove quickSwitcherKey from settingsAtom
- Remove Keyboard Shortcuts section from General settings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -27,7 +27,6 @@ import { useSelectedRoom } from '../../hooks/router/useSelectedRoom';
 import { useInboxNotificationsSelected } from '../../hooks/router/useInbox';
 import { useMediaAuthentication } from '../../hooks/useMediaAuthentication';
 import { usePresenceUpdater } from '../../hooks/usePresenceUpdater';
-import { QuickSwitcher } from '../../components/QuickSwitcher';
 
 function SystemEmojiFeature() {
   const [twitterEmoji] = useSetting(settingsAtom, 'twitterEmoji');
@@ -262,27 +261,6 @@ function MessageNotifications() {
   );
 }
 
-function QuickSwitcherFeature() {
-  const [open, setOpen] = useState(false);
-  const [quickSwitcherKey] = useSetting(settingsAtom, 'quickSwitcherKey');
-
-  useEffect(() => {
-    const handleKeyDown = (e: KeyboardEvent) => {
-      if ((e.ctrlKey || e.metaKey) && e.code === quickSwitcherKey) {
-        e.preventDefault();
-        setOpen((prev) => !prev);
-      }
-    };
-    window.addEventListener('keydown', handleKeyDown);
-    return () => {
-      window.removeEventListener('keydown', handleKeyDown);
-    };
-  }, [quickSwitcherKey]);
-
-  if (!open) return null;
-  return <QuickSwitcher onClose={() => setOpen(false)} />;
-}
-
 type ClientNonUIFeaturesProps = {
   children: ReactNode;
 };
@@ -296,7 +274,6 @@ export function ClientNonUIFeatures({ children }: ClientNonUIFeaturesProps) {
       <PresenceUpdater />
       <InviteNotifications />
       <MessageNotifications />
-      <QuickSwitcherFeature />
       {children}
     </>
   );