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.

src/index.html

@@ -582,6 +582,13 @@
                   <option value="landscape">Landscape</option>
                 </select>
               </div>
+              <!-- Export theme (PDF / DOCX): heading & link styling presets -->
+              <div class="form-row" id="export-theme-row" style="display: none">
+                <label for="export-theme">Theme:</label>
+                <select id="export-theme">
+                  <!-- Populated from src/main/ExportThemes.js at dialog open -->
+                </select>
+              </div>
               <div id="custom-page-size" class="form-row" style="display: none">
                 <label>Custom Size:</label>
                 <input