Read the locale where users can actually read it, and say which Stalwart answered

The account locale came from `x:Account/get`, which needs `sysAccountGet` — a
permission the built-in `user` role is not given, so the setting silently fell
back to the browser locale for exactly the people most likely to have set it.
Stalwart 0.16 carries the same field on `x:AccountSettings`, whose
`sysAccountSettingsGet` *is* part of that role. Both are now asked for in one
request and whichever answers wins, so admins and older servers keep working.

That pair of replies also says which generation we are talking to: only 0.16+
can parse the method name at all. About now reports that, plus the edition
from /api/account where the server offers it. It does not report a version
number because Stalwart does not publish one to clients — it hardcodes a
public "1.0.0" and keeps the real version to its SMTP internals — so the
screen says what was actually detected rather than inventing precision.

Also adds a light/dark toggle to the top bar, left of the settings button. The
stored setting is three-way, so the button acts on the theme actually on
screen: whichever one you see, a click gives you the other. Choosing "match
system" again stays in Settings › Appearance, where a three-way choice belongs.

web/src/store/settings.ts

@@ -1,3 +1,4 @@
+import { useEffect, useState } from "react";
 import { create } from "zustand";
 import { loadJson, saveJson } from "@/lib/storage";
 import { setDateTimePrefs, type DateFormat, type TimeFormat } from "@/lib/datetime";
@@ -186,6 +187,23 @@ if (typeof window !== "undefined") {
   window.matchMedia?.("(prefers-color-scheme: dark)").addEventListener("change", () => applyTheme());
 }
 
+/**
+ * The theme actually on screen, which is not the same as the setting: "system"
+ * resolves to whatever the OS is doing right now, and follows it as it changes.
+ */
+export function useEffectiveTheme(): "light" | "dark" {
+  const theme = useSettings((s) => s.settings.theme);
+  const [systemDark, setSystemDark] = useState(() => window.matchMedia?.("(prefers-color-scheme: dark)").matches ?? false);
+  useEffect(() => {
+    const mq = window.matchMedia?.("(prefers-color-scheme: dark)");
+    if (!mq) return;
+    const onChange = () => setSystemDark(mq.matches);
+    mq.addEventListener("change", onChange);
+    return () => mq.removeEventListener("change", onChange);
+  }, []);
+  return theme === "dark" || (theme === "system" && systemDark) ? "dark" : "light";
+}
+
 export const settings = () => useSettings.getState().settings;
 
 /**