fix: restore NotificationToggle component to SettingsPanel

Restores the NotificationToggle component that was incorrectly removed in commit 504f97c.
Without this component, users cannot enable/disable notifications via the UI, causing
notifications to fail even when browser permissions are granted.

Root cause: The component was removed as "unused" but is actually essential for controlling
the notificationStore.enabled flag that gates notification display on timer completion.

Changes:
- Add NotificationToggle import to SettingsPanel
- Place NotificationToggle at the top of settings options for visibility

Verified:
- Build successful (TypeScript, ESLint)
- UI tested on desktop (1280x720), tablet (768x1024), mobile (375x667)
- All breakpoints render correctly with proper touch targets

This fix enables users to control notifications through the settings panel.

Author: ryota-murakami

components/settings/SettingsPanel.tsx

@@ -7,6 +7,7 @@ import { useSettingsStore } from '@/lib/stores/settingsStore'
 import useStore from '@/lib/hooks/useStore'
 import { SoundSelector } from './SoundSelector'
 import { VolumeControl } from './VolumeControl'
+import { NotificationToggle } from './NotificationToggle'
 
 interface SettingsPanelProps {
   isOpen: boolean
@@ -44,6 +45,7 @@ export function SettingsPanel({ isOpen, onClose }: SettingsPanelProps) {
           {/* Settings Content - Scrollable */}
           <div className="flex-1 overflow-y-auto px-6">
             <div className="space-y-6">
+              <NotificationToggle />
               <SoundSelector value={soundPreset} onChange={setSoundPreset} />
               <VolumeControl value={volume} onChange={setVolume} />
             </div>