feat(companion): on-device room login with saved passwords

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>

release-notes.md

@@ -10,6 +10,7 @@
 - **Trail auto-pause** — recording **freezes on stops** (banking elapsed time and breaking the map line across the idle gap) and **resumes on movement** without ending the session; the home-screen blink keeps going while paused.
 - **Collapsible Tools** — tools are grouped into fold-in-place **Location / Comms / System** sections, the same model as Settings (Tools always opens folded to the section list), and the home carousel now uses **page-indicator icons** instead of dots.
 - **Waypoint coordinate editor** — add a waypoint by scroll-editing its latitude/longitude digit by digit.
+- **On-device room login with saved passwords** — log in to a room server straight from the device (no phone app): pick the room and the password prompt appears automatically (a blank password works for open rooms), or re-login any time via the room's **context-menu "Login…"**. The password is **remembered across reboots**, so a room you've used before logs back in without retyping; a failed login (e.g. the server's password changed) forgets the stale password so the next attempt prompts again. Saved passwords are written with the same atomic, crash-safe persistence as contacts and channels.
 
 ### Fixes