- Add `persistDeviceKey(w, handle)` to vault.ts: calls
`w.persist_device_key(handle)` and stores the ciphertext as base64
in chrome.storage.local `device_key_enc`.
- Wire into `registerDeviceAndPersistConfig` (create + attach flows)
so the key is persisted immediately after every `register_device`.
- Wire into `register_this_device` handler via `session.requireCurrent()`.
- Restore at unlock: if `device_key_enc` present, call
`w.restore_device_key(handle, blob)`; failure is caught + logged but
must not block unlock.
- Migration for pre-4.5b installs (absent `device_key_enc`): re-register,
persist new key, addDevice to remote (best-effort, must not block).
New device identity — org members must be re-granted to new pubkey.
- 4 new tests in device-key-persist.test.ts; existing 146 tests updated
and kept green; router.test.ts register_this_device stub extended.
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