feat(mobile): beacon task presence to suppress cross-device push fanout

[dmarticus]

Problem

After posthog/posthog#59313 (per-device presence beacon for push fanout), the server suppresses pushes about a task to any device with an active presence row for it. This PR is the mobile client half — it actually beacons.

Without this, the server-side filter has nothing to filter on and mobile keeps ringing for activity the user is already watching on another device.

Changes

• registerPushToken (src/lib/api.ts) now parses the id (UserPushToken.id UUID) from the registration response. Defensive parse so older server versions don't crash.
• pushTokenStore persists that UUID in SecureStore as deviceId alongside the existing token. Re-upload gate triggers when deviceId is missing, so existing users pull it on next foregrounding without re-registering.
• postTaskPresence / deleteTaskPresence in src/features/tasks/api.ts against POST/DELETE /api/projects/{team_id}/tasks/{task_id}/presence/ with { device_id }.
• useTaskPresence(taskId) hook in src/features/tasks/hooks/useTaskPresence.ts handles the lifecycle:
  • POST on mount when AppState === "active".
  • Refresh every 30s while mounted + foregrounded (server TTL is 60s).
  • DELETE on background/inactive, unmount, or task switch.
  • Waits for pushTokenStore.hydrate() so cold-start mounts don't miss the device_id.
  • Failures are intentionally swallowed — this is notification routing, not core functionality.
• app/task/[id].tsx calls useTaskPresence(taskId) once.

Out of scope / not done / needs decision

Desktop (apps/code) beacon

Desktop uses Electron's native Notification API (see apps/code/src/main/platform-adapters/electron-notifier.ts) — it never registers a push token with the server. So the server-side presence model, which keys on UserPushToken.id, has no row to write for a desktop session.

This means the original "I'm on my Mac, stop ringing my phone" complaint isn't fully fixed yet. Options to discuss:

• A. Server change: support free-form device_id on presence rows (decouple from UserPushToken). Desktop generates a stable UUID once, persists it in Electron's userData dir, and beacons normally. Cleanest model — presence shouldn't be a child of push token; one of those concepts is broader.
• B. Server change: let desktop register a "no-delivery" push token (platform: \"electron\") that the server flags as presence-only. Smaller server diff than A, hackier shape.
• C. Skip desktop presence entirely. The existing mobile-side mitigations from feat(mobile): hackathon mobile app #2237 (no awaitingPing arming from external user messages, 30s dedup, awaiting_user_input-only per-turn pings) already cover most desktop-driven activity. The leftover case is mobile-initiated work that completes while the user has moved to desktop.

My read is A is the right fix, but it's a server PR. Punting until we decide.

Inbox / new-task screens

• Inbox detail isn't task-scoped → no presence to claim.
• /task (new task composer) isn't viewing a task → no presence to claim.