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

src/renderer/App.tsx

@@ -1,6 +1,7 @@
 import { useState, useEffect } from 'react';
 import { AppShell } from './components/layout/AppShell';
 import { ModalLayer } from './components/modals/ModalLayer';
+import { CommandPalette } from './components/modals/CommandPalette';
 import { Toaster } from './components/ui/sonner';
 import { ReplPanel } from './components/tools/ReplPanel';
 import { PrintPreview } from './components/tools/PrintPreview';
@@ -30,6 +31,7 @@ function App() {
     <>
       <AppShell />
       <ModalLayer />
+      <CommandPalette />
       <Toaster />
       <UpdateBanner />
       <FirstRunWizard />