Language and theme follow the account: PATCH /me for the theme, /me's language by default

The server's /api/v2 now has PATCH /me {language, theme} and /me lists its languages and
themes. The theme is the account's and choosing one saves it there (a refusal is said);
a server whose /me lists no themes keeps the earlier choice in the app. The language is the
account's unless Settings chose this computer's or another (kept in the app, as the phone
does); a change of the account's opens the window again in it, once.

Fixtures re-pinned to the server's dev (f7f2ff2), with the new /api/v2 parity ones.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -16,9 +16,10 @@ All notable changes to LLeMbas Desktop. The format follows
   the account has another computer linked, a chip before the folder chooses it (this computer is
   the default). Its folder is one its agent chats have worked in, or a path typed in; the Terminal
   before the first message opens there too.
-- **Language and theme in Settings.** Under Appearance: the language (this computer's, or any of
-  the app's nine; the window opens again in it) and the theme (the account's, or Moria or Shire).
-  Both are kept in the app on this computer; the account's own stay as they are.
+- **Language and theme in Settings.** Under Appearance. The language is the account's, as the
+  server says it, unless this computer's or another of the app's nine is chosen (kept in the app;
+  the window opens again in it). The theme is the account's, and choosing one saves it there, so
+  the web interface and the phone follow; on a server that cannot be told, it is kept in the app.
 
 ### Fixed