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/state/settings.ts

@@ -82,7 +82,6 @@ export interface Settings {
   nightLightOpacity: number;
 
   deafenKey: string;
-  quickSwitcherKey: string;
 }
 
 const defaultSettings: Settings = {
@@ -134,7 +133,6 @@ const defaultSettings: Settings = {
   nightLightOpacity: 30,
 
   deafenKey: 'KeyM',
-  quickSwitcherKey: 'KeyP',
 };
 
 export const getSettings = (): Settings => {