- User-to-user PMs now use a server-derived shared room key (HMAC-SHA256)
instead of each user's personal PBKDF2 key (which differed per user,
making cross-user decryption impossible)
- Server sends room_key in pm_ready, pm_invite, and pm/history responses
- crypto.js: add importKeyBase64() for importing server-provided keys
- chat.js: use sharedKey for encrypt/decrypt in user-to-user PMs
- Violet AI transit encryption still uses personal key (unchanged)
- PM history decryption now handles errors gracefully per-message
- Encodes otherUser in history URL to prevent injection