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/search.json

@@ -0,0 +1,69 @@
+{
+  "q": "lembas",
+  "in": "all",
+  "offered": [
+    "chats",
+    "files",
+    "memories"
+  ],
+  "results": [
+    {
+      "kind": "chats",
+      "more": true,
+      "hits": [
+        {
+          "kind": "chat",
+          "title": "Waybread",
+          "snippet": "…how long does lembas keep & travel?",
+          "marks": [
+            [
+              15,
+              21
+            ]
+          ],
+          "archived": false,
+          "id": "chat1",
+          "agent": false
+        }
+      ]
+    },
+    {
+      "kind": "files",
+      "more": false,
+      "hits": [
+        {
+          "kind": "file",
+          "title": "bread.md",
+          "snippet": "/home/frodo/bread.md — Lembas recipe",
+          "marks": [
+            [
+              23,
+              29
+            ]
+          ],
+          "archived": false,
+          "path": "/home/frodo/bread.md"
+        }
+      ]
+    },
+    {
+      "kind": "memories",
+      "more": false,
+      "hits": [
+        {
+          "kind": "memory",
+          "id": "mem1",
+          "title": "",
+          "snippet": "Prefers lembas warm.",
+          "marks": [
+            [
+              8,
+              14
+            ]
+          ],
+          "archived": false
+        }
+      ]
+    }
+  ]
+}