feat: HEL-68 手机端 P0 同网址分流闸门与手机壳

在电脑端 index.html head 加极早分流闸门（?ui 覆盖优先，720px/移动 UA 判定，首屏前跳转 /m/）；新建 frontend/m/ 手机壳（index.html、tokens/shell CSS、api/session/router/boot JS、nav.config），首页五入口、占位图标页/功能页、登录页、日/夜主题与栈式返回。未改电脑 DOM/样式，未碰问天。

Co-authored-by: multica-agent <github@multica.ai>

frontend/index.html

@@ -5,6 +5,21 @@
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <meta name="color-scheme" content="light dark">
   <title>小白复盘</title>
+  <script>
+    (() => {
+      const params = new URLSearchParams(window.location.search);
+      const ui = params.get("ui");
+      const path = window.location.pathname;
+      const alreadyMobile = path === "/m" || path.indexOf("/m/") === 0;
+      const forcedMobile = ui === "mobile";
+      const forcedDesktop = ui === "desktop";
+      const autoMobile = window.matchMedia && window.matchMedia("(max-width: 720px)").matches;
+      const mobileUA = /Android|iPhone|iPad|iPod|Mobile|Windows Phone/i.test(navigator.userAgent || "");
+      if (!forcedDesktop && !alreadyMobile && (forcedMobile || autoMobile || mobileUA)) {
+        window.location.replace("/m/index.html" + window.location.search + window.location.hash);
+      }
+    })();
+  </script>
   <script>
     (() => {
       let theme = "light";