Fix account update method for unasaved accounts

If the account passed to `saved_accounts.update()` doesn't exist in the
config file, return instead of failing with a KeyError.

An account can be absent from the file if "Remember my account" was
unchecked on login, or if the file hasn't been updated yet on disk after
login (race condition).

TODO.md

@@ -1,5 +1,7 @@
 # TODO
 
+- fix device section headers when members pane is small
+- when promiting someone in room as admin, rank isn't updated in members list
 - save and restore status in accounts.json
 - mark accounts as offline when closing mirage
 - document new x11 dependnecy (auto-idle)

src/backend/user_files.py

@@ -206,10 +206,13 @@ class Accounts(JSONDataFile):
         presence: Optional[str] = None,
         order:    Optional[int] = None,
     ) -> None:
-        """Update existing account in the config and write to disk."""
+        """Update an account if found in the config file and write to disk."""
 
         saved = await self.read()
 
+        if user_id not in saved:
+            return
+
         if enabled is not None:
             saved[user_id]["enabled"] = enabled