The last Amber pain: a fresh launch re-decrypted the whole inbox (2 signer ops
per wrap). Decrypted messages are now persisted to IndexedDB
(src/lib/dmDecryptCache.ts, keyed viewerPk:wrapId, bounded 4000/account, pruned
oldest-first) and hydrated once per session before the first lookup, so a wrap is
decrypted at most once ever. Cleared per-account on logout (plaintext at rest
otherwise readable by another account on the device).
