fix: enable push notifications by mounting ServiceWorkerRegistration

## Problem
Push notifications were not working when timer completed - only sound would play.

## Root Cause
ServiceWorkerRegistration component existed but was never mounted in the app, causing:
1. Service Worker never registered → navigator.serviceWorker.ready hangs
2. Permission state not synced to Zustand store → always shows 'default'
3. Notification condition fails silently even if user granted permission

## Solution
Mount ServiceWorkerRegistration component in page.tsx to:
- Register Service Worker on app load
- Sync notification permission from browser to Zustand store
- Enable notifications to be displayed via Service Worker

## Changes
- app/[locale]/page.tsx: Import and render ServiceWorkerRegistration

## Verification
✅ Service Worker registration logs confirmed in console
✅ Build and lint passing
✅ Timer completion flow tested
✅ Ready for manual notification testing in real browser

## Testing Notes
Full notification testing requires manual browser testing (not headless):
1. Open in regular browser
2. Grant notification permission
3. Complete timer
4. Verify notification + sound both trigger

Author: ryota-murakami

app/[locale]/page.tsx

@@ -14,6 +14,7 @@ import { TimerControls } from '@/components/timer/TimerControls'
 import { TimeInput } from '@/components/timer/TimeInput'
 import { SettingsPanel } from '@/components/settings/SettingsPanel'
 import { LanguageToggle } from '@/components/LanguageToggle'
+import { ServiceWorkerRegistration } from '@/components/notifications/ServiceWorkerRegistration'
 import { NotificationTest } from '@/components/notifications/NotificationTest'
 
 export default function Home() {
@@ -115,6 +116,9 @@ export default function Home() {
 
   return (
     <main className="flex min-h-screen flex-col items-center justify-center gap-12 p-8">
+      {/* Service Worker Registration - runs once on mount */}
+      <ServiceWorkerRegistration />
+
       <div className="mx-auto w-full max-w-2xl space-y-12">
         {/* Header with Language and Settings */}
         <div className="relative text-center">