feat(ui): auto-enter room chat after a successful login

Logging in to a room server (first-time prompt or a remembered password) used
to leave the user on the room list, needing a second Enter to open the chat.
onRoomLoginResult() now opens the room history on success via a shared
openDmHistory() helper (extracted from the Enter-on-contact path).

The login result is async, so the auto-enter is gated on the user still being
on that room in the picker: phase CONTACT_PICK, room mode, no context menu /
share / pick-target sub-flow active, and the result pubkey matching the
selected contact. Otherwise it stays put (no yanking the user into a screen
they navigated away from).

Resolves the "auto-enter after login" code-review item. Builds green:
WioTrackerL1_companion_solo_dual.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/solo_features/message_screen/message_screen.md

@@ -42,9 +42,9 @@ Sensor placeholders appear automatically in the placeholder picker when the corr
 
 ### Rooms — logging in
 
-Posting to a **room server** requires a login handshake first, so the device can log in on its own — no phone app needed. The first time you press **Enter** on a room, a password prompt opens automatically; type the room's password and press the **✓** key (leave it empty and submit for open / no-password rooms).
+Posting to a **room server** requires a login handshake first, so the device can log in on its own — no phone app needed. The first time you press **Enter** on a room, a password prompt opens automatically; type the room's password and press the **✓** key (leave it empty and submit for open / no-password rooms). Once the login succeeds the room's chat **opens automatically** — no second Enter needed (as long as you're still on that room in the list).
 
-- **Passwords are remembered across reboots.** After a successful login the password is saved on the device, so picking that room again — even after a power cycle — logs back in silently without re-prompting.
+- **Passwords are remembered across reboots.** After a successful login the password is saved on the device, so picking that room again — even after a power cycle — logs back in silently and drops you straight into the chat.
 - **A wrong or changed password self-heals.** If a saved password stops working (e.g. the server's password was changed), the failed login forgets it, so the next **Enter** prompts you to type a new one.
 - **Re-login any time** with **Hold Enter** on the room → **Login…** (see the room context menu below) — useful to switch to a new password without waiting for a failure.
 - Passwords set from the **phone app** are saved on the device too, so it can post to that room standalone after a reboot.