Create crypto.ts with Web Crypto API utilities: PBKDF2 key derivation
(100K iterations, SHA-256), AES-256-GCM encrypt/decrypt, session key
management. Modify idb-storage.ts to transparently encrypt/decrypt
conversations when a session key is set. Disabled in dev mode via
VITE_DISABLE_CRYPTO=true.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>