feat: push notifications for messaging events

[andig]

fixes #16, pairs with evcc-io/evcc#31910, replaces #230

Real push notifications for evcc messaging events — delivered via APNs/FCM through the Expo push service, so they arrive with the app closed or in the background (addressing the feedback on #230).

• per-server "Send notifications to this device" checkbox in the server form; enabling requests notification permission, fetches the Expo push token and registers it with that server (POST /api/push/token)
• disabling the checkbox or deleting the server unregisters the token
• multiple servers register independently; message delivery itself happens server-side (see paired evcc PR)
• no in-app display logic needed — the OS shows the notification; foreground messages show as banner

TODO

• upload APNs key and FCM service account to the Expo project (EAS credentials) — required before pushes actually deliver
• F-Droid builds have no Play Services; token fetch fails gracefully but notifications won't work there

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