style: frosted glass sidebars (backdrop-blur-xl, bg-white/40)

Both sidebars get glassmorphism treatment so they stand out against
the scene background images.

frontend/src/App.tsx

@@ -136,7 +136,7 @@ export default function App() {
         {/* ── Main three-column body ── */}
         <div className="flex-1 flex min-h-0 px-4 gap-4">
           {/* LEFT: Focus Timer + Chat + Input */}
-          <div className="w-72 shrink-0 flex flex-col gap-3">
+          <div className="w-72 shrink-0 flex flex-col gap-3 bg-white/40 backdrop-blur-xl rounded-2xl p-3">
             <div className="shrink-0">
               <Timer />
             </div>
@@ -172,7 +172,7 @@ export default function App() {
           </div>
 
           {/* RIGHT: Ambient + Companions */}
-          <div className="w-64 shrink-0 flex flex-col">
+          <div className="w-64 shrink-0 flex flex-col bg-white/40 backdrop-blur-xl rounded-2xl p-3">
             <div className="flex-1 flex flex-col gap-3 overflow-y-auto">
               <MusicPlayer />
               <WhiteNoise />