backupAppStateToCloud

Back up host app state (≤2 KB; e.g. Kicks match witnesses) to the cloud, encrypted under a seed-derived key — no passkey/biometric, so it's safe to call automatically. No-op when no coordinator is wired. The coordinator hash-guards redundant uploads, so this is cheap to call after every meaningful state change. Never throws to the caller.

Pass null/empty for a "sigil exists, no app state" sentinel.