Gate push toggle on backend notification method

Add a GET /api/config endpoint exposing the front-end-facing backend
configuration (currently the notification delivery method, grouped under
a nested "notifications" object so the shape can grow). The settings
screen fetches it on init and only shows the per-device push toggle when
the backend method is "push" (and the platform supports push, which keeps
it off the browser).

Also fold in related push-notifications cleanups: fix the Android app
label ("frontend" -> "OOTT") so the notification permission dialog reads
correctly, remove the completed push_notifications.md plan, and update
TODO.md.

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

frontend/android/app/src/main/AndroidManifest.xml

@@ -1,6 +1,6 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
     <application
-        android:label="frontend"
+        android:label="OOTT"
         android:name="${applicationName}"
         android:icon="@mipmap/ic_launcher">
         <activity