feat(seo): add Dutch metadata, social previews, and JSON-LD

- Update all meta tags to Dutch (nl_NL locale)
- Add Open Graph and Twitter Card tags with image alt text
- Add JSON-LD structured data (WebApplication schema)
- Add favicon assets (svg, ico, png, apple-touch-icon)
- Add social preview image (og-image.png, 1200x630)
- Update theme color to #560fd9

src/frontend/favicon.svg

@@ -0,0 +1,5 @@
+<svg width="32" height="32" viewBox="0 0 32 32" fill="none" xmlns="http://www.w3.org/2000/svg">
+  <rect width="32" height="32" rx="6" fill="#6366f1"/>
+  <path d="M8 10L12 14L8 18" stroke="white" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M14 18H24" stroke="white" stroke-width="2.5" stroke-linecap="round"/>
+</svg>