Refactor: ES module architecture, tauri-plugin-store, unified design

Major changes:
- ES module system: main.js as central hub, shared modules
- tauri-plugin-store for persistent storage (dialog, fs plugins added)
- tauri-plugin-dialog for folder selection
- tauri-plugin-fs for real file system scanning
- Removed MOCK_FILES/FILE_TREE - real file scanner implemented
- All pages use unified page-header + page-content layout
- Custom titlebar with window controls
- Onboarding: 4-step flow with folder selection (Downloads default)
- CSS: unified design system, .hidden utility class added
- SVG icons throughout (no emoji)

Note: onboarding.js downloadDir() has runtime issue - needs fix

src/scripts/shared/titlebar.js

@@ -0,0 +1,60 @@
+export async function initTitlebar() {
+  const { getCurrentWindow } = await import('@tauri-apps/api/window');
+
+  const appWindow = getCurrentWindow();
+  const titlebar = document.getElementById('custom-titlebar');
+  const minimizeBtn = document.getElementById('titlebar-minimize');
+  const maximizeBtn = document.getElementById('titlebar-maximize');
+  const closeBtn = document.getElementById('titlebar-close');
+  const maximizeIcon = document.getElementById('maximize-icon');
+  const restoreIcon = document.getElementById('restore-icon');
+
+  async function updateIcon() {
+    const isMaximized = await appWindow.isMaximized();
+    maximizeIcon.style.display = isMaximized ? 'none' : 'block';
+    restoreIcon.style.display = isMaximized ? 'block' : 'none';
+  }
+
+  minimizeBtn.addEventListener('click', () => appWindow.minimize());
+
+  maximizeBtn.addEventListener('click', async () => {
+    await appWindow.toggleMaximize();
+    updateIcon();
+  });
+
+  closeBtn.addEventListener('click', () => appWindow.close());
+
+  titlebar.addEventListener('dblclick', async (e) => {
+    if (e.target.closest('.titlebar-controls')) return;
+    await appWindow.toggleMaximize();
+    updateIcon();
+  });
+
+  updateIcon();
+  appWindow.onResized(() => updateIcon());
+}
+
+export function mountTitlebar() {
+  const titlebarHTML = `
+    <div id="custom-titlebar" class="custom-titlebar" data-tauri-drag-region>
+      <div class="titlebar-left" data-tauri-drag-region>
+        <img src="../assets/logo.svg" alt="Chakmate" class="titlebar-logo" data-tauri-drag-region>
+        <span class="titlebar-title" data-tauri-drag-region>Chakmate</span>
+      </div>
+      <div class="titlebar-center" data-tauri-drag-region></div>
+      <div class="titlebar-controls">
+        <button id="titlebar-minimize" class="titlebar-btn" aria-label="최소화">
+          <svg width="12" height="12" viewBox="0 0 12 12"><rect y="5" width="12" height="2" fill="currentColor"/></svg>
+        </button>
+        <button id="titlebar-maximize" class="titlebar-btn" aria-label="최대화">
+          <svg id="maximize-icon" width="12" height="12" viewBox="0 0 12 12"><rect x="1" y="1" width="10" height="10" stroke="currentColor" stroke-width="2" fill="none"/></svg>
+          <svg id="restore-icon" width="12" height="12" viewBox="0 0 12 12" style="display:none"><path d="M3 1h8v8h-2v2H1V3h2V1zm1 3v5h5V4H4z" fill="currentColor"/></svg>
+        </button>
+        <button id="titlebar-close" class="titlebar-btn titlebar-btn-close" aria-label="닫기">
+          <svg width="12" height="12" viewBox="0 0 12 12"><path d="M1 1l10 10M11 1L1 11" stroke="currentColor" stroke-width="2"/></svg>
+        </button>
+      </div>
+    </div>
+  `;
+  document.body.insertAdjacentHTML('afterbegin', titlebarHTML);
+}