feat(export): themable PDF/Word exports; Windows CI green (v4.7.1)

Export themes:
- Six presets in the export dialog (basic + advanced modes) for PDF/DOCX:
  Default, Modern, Classic, Sepia, Minimal, Elegant
- PDF: LaTeX header (xcolor/titlesec) recolors headings, adds section
  rules and colored links — core-TeX packages only, hex-literal only
  (no injection surface)
- DOCX: styles.xml surgery recolors Heading1-6/Title/Subtitle/Hyperlink
  and swaps heading/body fonts; verified end-to-end against a real
  pandoc-produced docx
- Themes ride along in export presets (unknown ids fall back to Default)

Windows CI fixes:
- pdfjs standardFontDataUrl now a file:// URL (backslash paths failed
  pdfjs's trailing-slash validation, breaking extractText/extractImages)
- sharp temp cleanup EPERM retries; path-separator assertions; pdfjs
  test timeouts raised; batch suite testTimeout 30s

648/648 tests green; 4.7.1 linux+win artifacts rebuilt.

tests/main/ImageOperations.test.js

@@ -15,7 +15,15 @@ describe('ImageOperations', () => {
       .png()
       .toFile(inputPath);
   });
-  afterEach(() => fs.rmSync(tmpDir, { recursive: true, force: true }));
+  // Windows: sharp can hold the file handle briefly after await returns,
+// so plain rmSync throws EPERM — retry and tolerate stale temp dirs.
+afterEach(() => {
+  try {
+    fs.rmSync(tmpDir, { recursive: true, force: true, maxRetries: 5, retryDelay: 100 });
+  } catch {
+    /* leftover temp dir is cosmetic on locked-file platforms */
+  }
+});
 
   test('imageConvert converts PNG to JPEG', async () => {
     const outputPath = path.join(tmpDir, 'out.jpg');