[Feature] Add password protection for rooms

[kom50]

Summary

Allow room creators to set a password that others must enter to join.

Requirements

• Add "Set Password" option when creating room
• Show password modal when joining protected room
• Store hashed password on server
• Validate password before allowing join
• Show lock icon for protected rooms
• Allow room owner to change/remove password

UI/UX

• Password modal with input field and submit button
• Show error message for incorrect password
• Lock icon indicator in header for protected rooms
• Toast notification on successful unlock

Technical Notes

// Server-side room structure
{
  id: "room-abc123",
  passwordHash: "bcrypt-hash", // null if not protected
  owner: "socket-id",
  content: "...",
  users: Map
}

Security Considerations

• Use bcrypt for password hashing
• Rate limit password attempts (max 5 per minute)
• Clear password from memory after validation

Acceptance Criteria

• Room creator can set optional password
• Users must enter correct password to join
• Incorrect password shows error, doesn't allow entry
• Password can be changed by room owner