fix: show notification permission rationale in release builds

[sanasol]

The notification permission prompt was gated behind BuildConfig.DEBUG, so release users were never asked for notification permissions.

• Show rationale in all builds, not just debug
• Auto-register FCM token when permissions are already granted
• Check Google Play Services availability before FCM registration
• Add try-catch around subscribePush to prevent silent failures

[infi]

This is intentional as push notifications are (on the server side) not in a state where we can ship them to general availability.

Some feedback for the future:

• Please avoid using fully qualified package names when there are no conflicts. Use the auto import feature of relevant IDEs instead. Note that AI assistance tools may need to specifically get instructed to use imports instead of fully qualified package names.
• Please avoid using the android.util.Log class directly in new code. New code should use Square Logcat (logcat.logcat function).