Fix privacy atomicity, encryption, retry loop, and integrity verification

[Jambox11]

Summary

Four critical security and correctness fixes: real AES-256-GCM encryption for medical records, proper SHA-256 integrity hashing, enforced retry limits with exponential backoff, and atomic privacy settings saves.

Changes

#697 — Implement AES-256-GCM encryption for medical records

• Replaced Base64 encoding with authenticated encryption using Web Crypto API
• Properly consumes encryptionKey parameter in PBKDF2 key derivation
• IV generated per-encryption and stored with ciphertext for decryption
• Prevents exposure of plaintext medical data on IPFS

#698 — Use SHA-256 hash for on-chain integrity verification

• Replaced substring(0, 64) payload prefix with computeChecksum() (SHA-256 hex digest)
• Updated verifyRecord() to actually compare current data against stored hash
• Enables tamper detection for synced records

#699 — Fix retry loop to enforce maxRetries and add exponential backoff

• Thread result.attempts through recursive calls by passing existingResult to syncRecord()
• Permanently-failing records now stop after exactly 3 retries and reach status: 'failed'
• Exponential backoff: 2s, 4s, 6s delays across retry attempts

#696 — Make privacy settings save atomic with per-endpoint error reporting

• Update local state immediately after each successful API call (not after both succeed)
• Distinguish partial failures: "Visibility settings saved, but profile preferences failed"
• Prevents user mental model mismatch when one endpoint succeeds and the other fails

Testing notes

• Encryption: output not recoverable via atob() or Base64 decode
• Retry: simulate permanent failure and verify exactly 3 attempts, then terminal failed status
• Integrity: verify verifyRecord() detects tampering with off-chain data
• Privacy: test failure of 2nd API call and confirm 1st result is reflected in UI

Closes #696, Closes #697, Closes #698, Closes #699

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