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/components/modal.js

@@ -0,0 +1,46 @@
+// Modal Component
+
+export function showModal({ title, content, onConfirm, onCancel, confirmText = 'Confirm', cancelText = 'Cancel', danger = false }) {
+  const backdrop = document.createElement('div');
+  backdrop.className = 'modal-backdrop';
+  backdrop.innerHTML = `
+    <div class="modal">
+      <div class="modal-header">
+        <h3>${title}</h3>
+      </div>
+      <div class="modal-body">
+        ${content}
+      </div>
+      <div class="modal-footer">
+        <button class="btn btn-ghost" id="modal-cancel">${cancelText}</button>
+        <button class="btn ${danger ? 'btn-danger' : 'btn-primary'}" id="modal-confirm">${confirmText}</button>
+      </div>
+    </div>
+  `;
+  document.body.appendChild(backdrop);
+
+  backdrop.querySelector('#modal-cancel').onclick = () => {
+    backdrop.remove();
+    if (onCancel) onCancel();
+  };
+
+  backdrop.querySelector('#modal-confirm').onclick = () => {
+    backdrop.remove();
+    if (onConfirm) onConfirm();
+  };
+
+  backdrop.onclick = (e) => {
+    if (e.target === backdrop) {
+      backdrop.remove();
+      if (onCancel) onCancel();
+    }
+  };
+
+  return backdrop;
+}
+
+export function hideModal(backdrop) {
+  if (backdrop && backdrop.parentElement) {
+    backdrop.remove();
+  }
+}