Push: handle FCM messages — channels from notification preferences, deep links on tap #47

Part of #44 — Push notifications: real device push (FCM) to replace the WorkManager poll

• A FirebaseMessagingService that raises notifications through the existing channel and
  deep-link plumbing built for the poll (il_mentions et al., grouped + capped, last-seen
  marker) — this issue should reuse that code, not duplicate it.
• Route each notification to its category channel per the account's notification-preferences
  push settings.
• Tap → the same deep-link destinations the poll notifications already use.
• Handle foreground vs background delivery, and de-duplicate against anything the poll may have
  already shown during the transition.

Tests: payload → channel mapping; tap → route; no duplicate notification when both paths see
the same item.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.