Add new-mail notifications and persisted settings

Increment 6 — notifications and settings.

- Local new-mail notifications (no push service): MailNotifier posts a notification
  when background sync finds newly-arrived unread mail, and tapping it opens the app.
  Adds a POST_NOTIFICATIONS request on launch.
- MailSyncer detects genuinely new messages (diff against cached ids, skipped on an
  account's first sync) and notifies when the setting is enabled.
- SettingsRepository (Preferences DataStore) persists settings; the Settings screen
  gains a Notifications section, and "Use wallpaper colors" now actually drives the
  Material You theme (MainActivity collects it reactively).
- assemble/test/lint green; verified on the Android 17 emulator — delivered a new
  message and the on-device notification appeared in the shade.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/AndroidManifest.xml

@@ -5,6 +5,7 @@
 
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.READ_CONTACTS" />
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
 
     <application
         android:name=".LibreMailApplication"