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>

tests/ui/fixtures/chat.json

@@ -0,0 +1,28 @@
+{
+  "id": "chat1",
+  "kind": "chat",
+  "title": "Waybread",
+  "model": "qwen",
+  "folder_id": null,
+  "pinned": false,
+  "archived": false,
+  "temporary": false,
+  "unread": true,
+  "running": false,
+  "mode": "",
+  "effort": "low",
+  "device": null,
+  "device_session_id": null,
+  "parent_chat_id": null,
+  "agents": {
+    "min": 1,
+    "max": 4
+  },
+  "subagents": {
+    "offered": true,
+    "named_only": false,
+    "cap": 32
+  },
+  "created_at": "2026-01-02T03:04:05+00:00",
+  "updated_at": "2026-01-02T03:04:05+00:00"
+}