docs: add mdbook language switcher script

docs/shared/lang-switcher.js

@@ -0,0 +1,47 @@
+// Injects a language toggle (EN / 中文) into the mdBook menu bar.
+// Swaps the leading /en-US/ <-> /zh-CN/ path segment, preserving the
+// rest of the path; falls back to the language root on 404 navigation.
+(function () {
+  const LANGS = [
+    { code: "en-US", label: "EN" },
+    { code: "zh-CN", label: "中文" },
+  ];
+
+  function currentLang() {
+    const m = window.location.pathname.match(/\/(en-US|zh-CN)\//);
+    return m ? m[1] : "en-US";
+  }
+
+  function swapTo(code) {
+    const path = window.location.pathname;
+    const cur = currentLang();
+    if (path.includes(`/${cur}/`)) {
+      return path.replace(`/${cur}/`, `/${code}/`);
+    }
+    return `/${code}/`;
+  }
+
+  function build() {
+    const right = document.querySelector(".right-buttons");
+    if (!right) return;
+    const cur = currentLang();
+    const wrap = document.createElement("div");
+    wrap.className = "lang-switcher";
+    wrap.style.display = "inline-flex";
+    wrap.style.gap = "0.5rem";
+    wrap.style.marginInlineStart = "0.5rem";
+    LANGS.forEach((l) => {
+      const a = document.createElement("a");
+      a.textContent = l.label;
+      a.href = swapTo(l.code);
+      a.title = l.code;
+      a.setAttribute("aria-current", l.code === cur ? "true" : "false");
+      if (l.code === cur) a.style.fontWeight = "bold";
+      wrap.appendChild(a);
+    });
+    right.appendChild(wrap);
+  }
+
+  if (document.readyState !== "loading") build();
+  else document.addEventListener("DOMContentLoaded", build);
+})();