Show sealed previews on Android, opened by the app itself

[sivert-io]

Android notifications show who wrote, where, and the first line, the same as on iPhone (GRYT-1698).

How. The relay (push#4) sends a phone that registered with opens: true data-only messages. Android doesn't show those by itself. It wakes the app, which builds the notification:

• index.ts loads src/notify/androidPushTask.ts before the router. It defines a task for expo-notifications' registerTaskAsync, and Android runs it for a data-only push with the app in the foreground, the background or closed.
• The task finds the server's preview key by the relay's tag, opens the preview with openPreview, and shows a local notification: who wrote as the title, where as the subtext, the line as the body, and the tag in its data so a tap opens the right server. If it can't open the preview, it shows the relay's fixed text instead.
• openPreview reads the same format as the iPhone extension (AES-256-GCM, gryt-push-1|<tag> as additional data), using @noble/ciphers. Its test seals the way the server does.

Registration. Android now registers with opens: true. A capability made before this was registered without it, so the phone swaps it once (opens in the push state). That also means a new preview key, which the server picks up on the next push:register.

New dependencies: expo-task-manager ~57.0.21 (a native module, so this needs a new build) and @noble/ciphers ^2.4.0.

Tested on the Android emulator (Pixel 7, API 36) against the live relay with push#4 deployed, a local server and the web client:

• App in the background (dev build): the notification read Cinder, subtext #General · Gryt Blog Demo, text Android preview test: who, where and this line.
• App closed (release build, process killed): Android started it, the task ran headless in about a second, and the same notification showed for Third test, release build, app closed.
• The relay stored the phone with opens = 1.

Look at:

• With the app closed, the dev build's launcher crashes on start (SplashScreenManager not found) before the task can run, so test that case with a release build.
• expo-task-manager is required inside a try, on Android only, so an iPhone or an old dev client without the native module doesn't crash on start.
• A force-stopped app ("Force stop" in Android settings) gets no FCM at all, same as any app.

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