fix: harden cryptographic session lifecycle and integrity validation

[Temi709]

Summary

This PR strengthens the security and reliability of the messaging and file handling pipeline by addressing multiple cryptographic and integrity issues.

Changes

• Corrected ECDH session establishment to derive shared secrets using the local private key and peer public key with the proper WebCrypto API usage.
• Persisted device identity private CryptoKey objects in IndexedDB via structured cloning to maintain stable device identities across sessions and page reloads.
• Unified push notification recipient filtering so both text and file message push paths consistently honor conversation mute status and device pushEnabled preferences.
• Added SHA-256 integrity verification during file upload confirmation to detect corrupted or substituted uploads before marking files as ready.

Testing

• Added/updated unit tests covering:
  • Two-party ECDH shared secret derivation.
  • Identity key persistence across simulated page reloads.
  • Consistent push recipient filtering across all push paths.
  • File integrity verification for matching and mismatched hashes.
• Verified no regressions to encrypted messaging, push delivery behavior, or file upload workflows.

Closes #348
Closes #350
Closes #351
Closes #352

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