feat: add PDF and PowerPoint export

- Install pptxgenjs@4.0.1 for client-side PPTX generation
- Add exportPDF() (window.print) and exportPPTX(lang) in src/data/export.js
- Add ExportMenu dropdown component in src/components/ExportMenu.jsx
- Mount ExportMenu in AppHeader next to the language selector
- Add NL/EN/FR i18n strings for the export feature
- Add @media print CSS to hide chrome when printing to PDF
- Add 8 tests covering exportPDF, exportPPTX (all languages), and ExportMenu

Closes #19

Author: Copilot

GRIP-Visualizer.pptx

@@ -16,6 +16,7 @@
     "test:e2e:ui": "playwright test --ui"
   },
   "dependencies": {
+    "pptxgenjs": "^4.0.1",
     "react": "19.2.7",
     "react-dom": "19.2.7"
   },