frontend: redesign homepage with magic dock and login dialog

- Replace top login/register buttons with magic dock navigation
- Add dock items: home, downloads, blog, and conditional login/logout
- Show dashboard icon in dock when authenticated
- Extract HomePageClient for client-side dialog state

frontend/app/home-dock.tsx

@@ -0,0 +1,74 @@
+"use client";
+
+import { Dock, DockIcon } from "@/components/ui/dock";
+import { signOut } from "next-auth/react";
+import { Home, User, LogOut, Download, BookOpen, LayoutDashboard } from "lucide-react";
+
+export function HomeDock({
+  isAuthenticated,
+  onLoginClick,
+}: {
+  isAuthenticated: boolean;
+  onLoginClick?: () => void;
+}) {
+  return (
+    <div className="fixed bottom-8 left-1/2 -translate-x-1/2">
+      <Dock>
+        <DockIcon>
+          <a href="/" aria-label="首页">
+            <Home className="h-5 w-5 text-slate-700" />
+          </a>
+        </DockIcon>
+        <DockIcon>
+          <a
+            href="https://file.liukersun.com"
+            target="_blank"
+            rel="noopener noreferrer"
+            aria-label="下载网站"
+          >
+            <Download className="h-5 w-5 text-slate-700" />
+          </a>
+        </DockIcon>
+        <DockIcon>
+          <a
+            href="https://blog.liukersun.com"
+            target="_blank"
+            rel="noopener noreferrer"
+            aria-label="博客"
+          >
+            <BookOpen className="h-5 w-5 text-slate-700" />
+          </a>
+        </DockIcon>
+        {isAuthenticated && (
+          <DockIcon>
+            <a href="/dashboard" aria-label="仪表盘">
+              <LayoutDashboard className="h-5 w-5 text-slate-700" />
+            </a>
+          </DockIcon>
+        )}
+        <DockIcon>
+          {isAuthenticated ? (
+            <button
+              onClick={async () => {
+                await signOut({ redirect: false });
+                window.location.href = "/";
+              }}
+              aria-label="退出"
+              className="flex h-full w-full items-center justify-center"
+            >
+              <LogOut className="h-5 w-5 text-slate-700" />
+            </button>
+          ) : (
+            <button
+              onClick={onLoginClick}
+              aria-label="登录"
+              className="flex h-full w-full items-center justify-center"
+            >
+              <User className="h-5 w-5 text-slate-700" />
+            </button>
+          )}
+        </DockIcon>
+      </Dock>
+    </div>
+  );
+}