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/push_schedule.json

@@ -0,0 +1,34 @@
+{
+  "schedule_id": "sched1",
+  "run_id": "run1",
+  "chat_id": "chat1",
+  "status": "ok",
+  "visibility": "private",
+  "actions": [
+    "open",
+    "run_again"
+  ],
+  "ask": "",
+  "notification": {
+    "title": "Waybread check",
+    "body": "All the waybread is wrapped.",
+    "public": {
+      "title": "Waybread check",
+      "body": "Finished"
+    },
+    "actions": [
+      "open",
+      "run_again"
+    ],
+    "labels": {
+      "open": "Open",
+      "run_again": "Run again"
+    },
+    "ask": ""
+  },
+  "pause_label": "Pause",
+  "title": "LLeMbas",
+  "body": "A scheduled task has finished.",
+  "url": "/scheduled/sched1?run=run1",
+  "kind": "schedule"
+}