feat: add frontend UI for SimpleNote Web

- Vanilla JS frontend with dark theme
- Dashboard with sidebar (libraries tree, tags), document grid, search
- Document viewer with markdown rendering and metadata panel
- Document editor with split write/preview and formatting toolbar
- Login screen with token authentication
- All styled according to UI/UX specs (dark theme, accent #00d4aa)
- API client for all endpoints
- Responsive design

public/js/views/login.js

@@ -0,0 +1,29 @@
+// Login View
+
+export function renderLogin({ onLogin }) {
+  return `
+    <div class="login-screen">
+      <div class="login-card">
+        <h1>📝 SimpleNote</h1>
+        <p>Enter your API token to continue</p>
+        <form class="login-form" id="login-form">
+          <div class="form-group">
+            <input type="password" id="token-input" placeholder="API Token" autocomplete="off" required>
+          </div>
+          <p class="login-error" id="login-error">Invalid token. Please try again.</p>
+          <button type="submit" class="btn btn-primary" style="width:100%">Login</button>
+        </form>
+      </div>
+    </div>
+    <script>
+      document.getElementById('login-form').onsubmit = async (e) => {
+        e.preventDefault();
+        const token = document.getElementById('token-input').value;
+        const error = await onLogin(token);
+        if (error) {
+          document.getElementById('login-error').classList.add('visible');
+        }
+      };
+    </script>
+  `;
+}