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/features/settings/general/General.tsx

@@ -1316,43 +1316,6 @@ function Messages() {
   );
 }
 
-function KeyboardShortcuts() {
-  const [quickSwitcherKey, setQuickSwitcherKey] = useSetting(settingsAtom, 'quickSwitcherKey');
-  const qsBind = useKeyBind(setQuickSwitcherKey);
-
-  return (
-    <Box direction="Column" gap="100">
-      <Text size="L400">Keyboard Shortcuts</Text>
-      <SequenceCard
-        className={SequenceCardStyle}
-        variant="SurfaceVariant"
-        direction="Column"
-        gap="400"
-      >
-        <SettingTile
-          title="Quick Room Switcher"
-          description="Open the quick room switcher overlay. Ctrl/Cmd + this key."
-          after={
-            <Button
-              size="300"
-              variant={qsBind.listening ? 'Warning' : 'Secondary'}
-              fill={qsBind.listening ? 'Solid' : 'Soft'}
-              radii="300"
-              outlined
-              onClick={qsBind.startListening}
-              style={{ minWidth: '90px' }}
-            >
-              <Text size="B300">
-                {qsBind.listening ? 'Press a key…' : `Ctrl+${keyLabel(quickSwitcherKey)}`}
-              </Text>
-            </Button>
-          }
-        />
-      </SequenceCard>
-    </Box>
-  );
-}
-
 type GeneralProps = {
   requestClose: () => void;
 };
@@ -1383,7 +1346,6 @@ export function General({ requestClose }: GeneralProps) {
               <Messages />
               <Privacy />
               <Calls />
-              <KeyboardShortcuts />
             </Box>
           </PageContent>
         </Scroll>