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/markdown-extensions.test.js

@@ -135,7 +135,7 @@ describe('Markdown Extensions', () => {
         tokens: [{ type: 'text', text: 'Be careful!' }],
       };
       const mockParser = {
-        parse: jest.fn((tokens) => '<p>Be careful!</p>'),
+        parse: jest.fn((_tokens) => '<p>Be careful!</p>'),
       };
       const context = { parser: mockParser };
       const html = extension.renderer.call(context, token);