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/files_listing.json

@@ -0,0 +1,22 @@
+{
+  "path": "/home/frodo",
+  "parent": "/home",
+  "entries": [
+    {
+      "name": "recipes",
+      "path": "/home/frodo/recipes",
+      "kind": "dir",
+      "size": 0,
+      "mime": "",
+      "updated_at": "2026-01-02T03:04:05+00:00"
+    },
+    {
+      "name": "lembas.md",
+      "path": "/home/frodo/lembas.md",
+      "kind": "file",
+      "size": 27,
+      "mime": "text/markdown",
+      "updated_at": "2026-01-02T03:04:05+00:00"
+    }
+  ]
+}