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/me.json

@@ -0,0 +1,57 @@
+{
+  "api": 2,
+  "version": "2.0.0",
+  "account": {
+    "id": "user1",
+    "name": "Frodo",
+    "email": "frodo@shire.test"
+  },
+  "language": "sk",
+  "theme": "shire",
+  "default_model": "qwen",
+  "permissions": {
+    "chat.create": true,
+    "chat.delete": false,
+    "chat.model_select": true,
+    "chat.system_prompt": true,
+    "chat.params": true,
+    "files.upload": true,
+    "tools.agent": true,
+    "audio.transcribe": true,
+    "audio.listen": true
+  },
+  "services": {
+    "stt": true,
+    "tts": false,
+    "search": true,
+    "fetch": true,
+    "images": false
+  },
+  "scopes": [
+    "app",
+    "models"
+  ],
+  "language_setting": "sk",
+  "languages": [
+    {
+      "id": "en",
+      "name": "English"
+    },
+    {
+      "id": "sk",
+      "name": "Slovenčina"
+    }
+  ],
+  "themes": [
+    {
+      "id": "moria",
+      "label": "Moria",
+      "scheme": "dark"
+    },
+    {
+      "id": "shire",
+      "label": "Shire",
+      "scheme": "light"
+    }
+  ]
+}