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.

server/src/accountinfo.test.ts

@@ -0,0 +1,51 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { interpretAccountInfo } from "./upstream.js";
+
+/**
+ * The account locale used to be read only from `x:Account/get`, which needs
+ * the `sysAccountGet` permission — one the built-in `user` role is not given.
+ * Ordinary users therefore silently fell back to the browser locale. Stalwart
+ * 0.16 exposes the same field on `x:AccountSettings`, which users *can* read,
+ * so both are asked for and whichever answers wins.
+ */
+
+type Responses = [string, Record<string, unknown>, string][];
+
+const settingsOk = (locale: string): Responses[number] => ["x:AccountSettings/get", { list: [{ id: "singleton", locale }] }, "s"];
+const accountOk = (locale: string): Responses[number] => ["x:Account/get", { list: [{ id: "a1", locale }] }, "a"];
+const failed = (id: string, type: string): Responses[number] => ["error", { type }, id];
+
+test("prefers the locale a regular user is allowed to read", () => {
+  const info = interpretAccountInfo([settingsOk("de_DE.UTF-8"), accountOk("fr_FR")]);
+  assert.equal(info.locale, "de-DE");
+  assert.equal(info.generation, "0.16+");
+});
+
+test("falls back to x:Account when the settings object is forbidden", () => {
+  const info = interpretAccountInfo([failed("s", "forbidden"), accountOk("sr_RS@latin")]);
+  assert.equal(info.locale, "sr-Latn-RS");
+});
+
+test("an older server is recognised by its unknownMethod, and still yields a locale", () => {
+  const info = interpretAccountInfo([failed("s", "unknownMethod"), accountOk("en_GB")]);
+  assert.equal(info.generation, "pre-0.16");
+  assert.equal(info.locale, "en-GB");
+});
+
+test("a server answering the new method is 0.16+ even with no locale set", () => {
+  const info = interpretAccountInfo([["x:AccountSettings/get", { list: [] }, "s"], failed("a", "forbidden")]);
+  assert.equal(info.generation, "0.16+");
+  assert.equal(info.locale, null);
+});
+
+test("neither answering leaves everything unknown rather than guessing", () => {
+  const info = interpretAccountInfo([failed("s", "forbidden"), failed("a", "forbidden")]);
+  assert.deepEqual(info, { locale: null, generation: null, edition: null });
+  assert.deepEqual(interpretAccountInfo([]), { locale: null, generation: null, edition: null });
+});
+
+test("locales that carry no language are dropped, not passed through", () => {
+  assert.equal(interpretAccountInfo([settingsOk("C")]).locale, null);
+  assert.equal(interpretAccountInfo([settingsOk("POSIX")]).locale, null);
+});