feat(voice): SSE bridge + Siri-blob overlay for wake-word script

Adds the tablet side of voice assistant integration. External Python
script (openWakeWord + Groq STT + OpenClaw) will POST state transitions
to /api/voice/event with a bearer token, and the tablet shows a
fullscreen overlay with Siri-style animated blob + current agent +
recognized text / response text.

- lib/voice-bus.ts — in-process EventEmitter singleton, preserved
  across hot reloads via globalThis
- app/api/voice/event — POST, bearer-auth via VOICE_API_KEY env,
  validates event kind, broadcasts on voiceBus
- app/api/voice/stream — GET, SSE endpoint, per-connection listener
  with 15s keep-alive ping and abort-signal cleanup
- components/VoiceOverlay — full-screen overlay, 3-layer pulsing
  Siri blob, per-agent palette (cosmo indigo/violet, lusya pink/rose),
  auto-dismiss timeouts (wake=20s safety, response=6s, error=4s),
  auto-reconnect on SSE drop
- middleware bypasses /api/voice/event so the script does not need
  a user auth cookie
- VoiceOverlay mounted in HomePageInner outside tab routing so it
  appears on every view

app/page.tsx

@@ -11,6 +11,7 @@ import CalendarTab from '@/components/CalendarTab'
 import NotesTab from '@/components/NotesTab'
 import TransportWidget from '@/components/TransportWidget'
 import WeatherAnimation from '@/components/WeatherAnimation'
+import VoiceOverlay from '@/components/VoiceOverlay'
 
 type Tab = 'home' | 'devices' | 'calendar' | 'notes' | 'settings'
 
@@ -1107,6 +1108,8 @@ function HomePageInner() {
         )}
       </AnimatePresence>
 
+      <VoiceOverlay />
+
       <Sidebar active={tab} onChange={setTab} />
 
       {/* Night-shift warm tint overlay */}