feat: add frontend dashboard, login page and portal entry

Public directory contains the Kanban SPA with dashboard UI, login
page, and styles. Portal directory provides a standalone service
entry page with CSS styling.

public/login.js

@@ -0,0 +1,18 @@
+const form = document.getElementById('loginForm');
+const tokenInput = document.getElementById('tokenInput');
+const loginError = document.getElementById('loginError');
+
+form.addEventListener('submit', async (event) => {
+  event.preventDefault();
+  loginError.textContent = '';
+  const response = await fetch('/api/auth/login', {
+    method: 'POST',
+    headers: { 'content-type': 'application/json' },
+    body: JSON.stringify({ token: tokenInput.value })
+  });
+  if (response.ok) {
+    window.location.href = '/';
+    return;
+  }
+  loginError.textContent = response.status === 429 ? '尝试次数过多' : 'Token 无效';
+});