fix: cross-browser sync import corrupting local encryption + v2.0.9

Root cause: when importing an encrypted sync code from another device,
_decryptFromSync was replacing the local encryption config (salt) with
the source device's salt. This caused:
1. Local key cache derived from wrong salt
2. Local data (encrypted with local salt) became unreadable
3. _applyData tried to write with the wrong key

Fixed with a complete refactor of cross-device decryption:
- authenticateForSync() derives a TEMPORARY key using the source salt
- Temporary key stored separately as 'tempSyncKey' in IndexedDB
- Local encryption config and cached key are NEVER modified
- After decryption, _applyData writes via _writeSecure using the
  LOCAL key (which uses the local salt)
- _handleDecryptedMeta extracted for code reuse

Also:
- Options.js auth handler detects pending sync import and routes to
  authenticateForSync instead of regular authenticate
- After auth success, automatically retries the import
- README updated: imported passwords are protected (dots in UI,
  vault password to reveal, AES-256 encrypted at rest)
- Bumped to v2.0.9

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

src/options/options.html

@@ -591,7 +591,7 @@
     </section>
 
     <footer>
-      <p>Silent Send v2.0.7</p>
+      <p>Silent Send v2.0.9</p>
     </footer>
   </div>