fix(mobile): reconcile presence snapshots after reconnect

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Summary

• Query the relay's current presence snapshot when mobile begins tracking users, reconnects, and periodically while connected.
• Resolve relay-signed snapshot subjects from the p tag, with the event pubkey as a fallback.
• Mark tracked users absent from a snapshot as offline without overwriting a live event that arrived while the query was in flight.
• Batch and deduplicate snapshot requests, and ignore stale responses from an earlier connection generation.
• Preserve the existing live kind-20001 subscription for immediate updates.

Presence events are ephemeral, so stored-event backfill cannot recover updates missed while Android is backgrounded or reconnecting. Mobile previously depended only on live presence events; a missed event could therefore leave the displayed status stale indefinitely. This adds the snapshot reconciliation path already needed for ephemeral state while retaining live updates.

Related issue

Related to #4402, which reports message catch-up after Android backgrounding. This PR addresses the separate presence-reconciliation symptom and does not claim to fix stored-message catch-up.

Testing

• dart format --output=none --set-exit-if-changed on both changed files: passed.
• Focused presence provider suite: 20 tests passed.
• Repository-required . ./bin/activate-hermit && just ci: passed, including 1,177 mobile tests.
• Tested on a real Android device: the observed stale/offline presence symptom was resolved.
• Message catch-up behavior remains unconfirmed and out of scope for this PR.

No visual layout was changed, so before/after screenshots are not applicable.