feat(white-noise): add Web Audio generated white/pink/brown/rain/cafe noise player

Separate from lofi music per original spec. Toggleable, volume control, always available in focus column.
Finishes item 5.

frontend/src/App.tsx

@@ -4,6 +4,7 @@ import BackgroundScene from './components/BackgroundScene';
 import MusicPlayer from './components/MusicPlayer';
 import Timer from './components/Timer';
 import Notes from './components/Notes';
+import WhiteNoise from './components/WhiteNoise';
 import KiraAvatar from './components/KiraAvatar';
 import ChatBubble from './components/ChatBubble';
 import PetZone from './components/PetZone';
@@ -136,11 +137,12 @@ export default function App() {
               />
             </div>
 
-            {/* Column 2: Timer + Music */}
+            {/* Column 2: Timer + Music + Notes + WhiteNoise */}
             <div className="space-y-4">
               <Timer />
               <MusicPlayer />
               <Notes />
+              <WhiteNoise />
             </div>
 
             {/* Column 3: Chat + Text Input */}