docs(book): correct lang-switcher LOCALES comment + reorder to match locales.toml

The header comment claimed `cargo mdbook build` would regenerate this
file from locales.toml at the repo root. That xtask subcommand was
removed in a later refactor, so the file is now hand-maintained, but
the misleading comment caused multiple PRs to either ignore manual
edits as drive-by drift or treat the file as authoritative when it had
silently desynced from locales.toml (this file: en/ja/fr/es/zh-CN;
locales.toml: en/fr/ja/es/zh-CN).

Drop the generator claim, replace with an honest "hand-maintained,
keep in sync with locales.toml" instruction, and reorder LOCALES to
match locales.toml so the two lists agree.

Author: singlerider

docs/book/theme/lang-switcher.js

@@ -4,13 +4,17 @@
 // matches one of LOCALES, then renders a dropdown linking to the same page
 // in every other locale.
 //
-// LOCALES is generated from locales.toml at build time by `cargo mdbook build`.
-// Edit locales.toml at the repo root to add or remove locales.
+// LOCALES is hand-maintained. The previous comment claimed an
+// `cargo mdbook build` step would regenerate it from locales.toml at the
+// repo root, but that xtask command no longer exists, so the two lists
+// drifted (this file ordered en/ja/fr/es/zh-CN; locales.toml ordered
+// en/fr/ja/es/zh-CN). Keep the order below in sync with locales.toml
+// when adding or removing a locale.
 (function () {
   const LOCALES = [
     { code: "en", label: "English" },
-    { code: "ja", label: "日本語" },
     { code: "fr", label: "Français" },
+    { code: "ja", label: "日本語" },
     { code: "es", label: "Español" },
     { code: "zh-CN", label: "中文" },
   ];