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/folders.json

@@ -0,0 +1,20 @@
+{
+  "folders": [
+    {
+      "id": "fold1",
+      "name": "Waybread",
+      "parent_id": null,
+      "kind": "",
+      "position": 0,
+      "collapsed": false
+    },
+    {
+      "id": "fold2",
+      "name": "Recipes",
+      "parent_id": "fold1",
+      "kind": "chat",
+      "position": 1,
+      "collapsed": true
+    }
+  ]
+}