feat: enhance WelcomeDialog, export dialogs, GitStatusPanel, add CommandPalette, FindReplaceBar, and new converter dialogs

- WelcomeDialog: add quick start, feature showcase, keyboard shortcuts, recent files, version display
- ExportDocxDialog/ExportHtmlDialog/ExportPdfDialog: add header/footer, paper size, margin controls
- GitStatusPanel: full staging/unstaging, discard, commit workflow
- CommandPalette: fuzzy command search with keyboard navigation
- FindReplaceBar: find/replace with regex, case, whole-word options
- New dialogs: BatchMediaConverterDialog, HeaderFooterDialog, PdfEditorDialog, UniversalConverterDialog
- Tests: comprehensive coverage for all new/updated components

tests/sidebar.test.js

@@ -30,7 +30,7 @@ describe('SidebarManager', () => {
   });
 
   test('starts collapsed', () => {
-    const mgr = new SidebarManager();
+    new SidebarManager();
     expect(document.getElementById('sidebar').classList.contains('collapsed')).toBe(true);
   });