Log in to a room server from the device UI with no phone app: picking a
room prompts for its password (blank allowed for open rooms), and a
context-menu "Login..." allows re-login. Successful passwords are
persisted to a dedicated /room_pw file so a previously-used room logs
back in after reboot without retyping; a failed login forgets the
(now-stale) saved password so the next attempt prompts again.
The room-password file is written via a temp file + atomic rename
(new DataStore::commitFile helper), matching the crash-safety of
contacts/channels persistence.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>