API keys for external providers (OpenAI, Mistral, etc.) were stored as
plaintext in localStorage, vulnerable to browser extensions and XSS.
Add password-derived AES-256-GCM encryption: on login the user's password
is used via PBKDF2 (100k iterations, SHA-256) to derive an in-memory
encryption key. API keys are encrypted before writing to localStorage and
decrypted on read. The derived key is never persisted — cleared on logout,
re-derived on next login.
Legacy plaintext keys are transparently migrated on first access. Password
changes re-encrypt all stored keys. No backend changes required — the
existing RSA-OAEP transit encryption is unaffected.
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