refactor: extract shared partials from HTML pages (nav, footer, styles, modals)

- Created build-time templating system using existing build-html.cjs
- Extracted index.html into source template with partials:
  _styles_index.html, _nav_index.html, _modals.html
- All 4 templated pages (index, impressum, privacy, terms) use partials
- docs.html excluded (Swagger UI, completely different structure)
- Added HTML build step to Dockerfile
- Built output is byte-identical to original files

public/index.html

@@ -17,7 +17,7 @@
 <link rel="preconnect" href="https://fonts.googleapis.com">
 <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
 <script type="application/ld+json">
-{"@context":"https://schema.org","@type":"SoftwareApplication","name":"DocFast","url":"https://docfast.dev","applicationCategory":"DeveloperApplication","operatingSystem":"Web","description":"Convert HTML and Markdown to beautiful PDFs with a simple API call. Fast, reliable, developer-friendly.","offers":[{"@type":"Offer","price":"0","priceCurrency":"EUR","name":"Free","description":"100 PDFs/month"},{"@type":"Offer","price":"9","priceCurrency":"EUR","name":"Pro","description":"2,500 PDFs per month","billingIncrement":"P1M"}]}
+{"@context":"https://schema.org","@type":"SoftwareApplication","name":"DocFast","url":"https://docfast.dev","applicationCategory":"DeveloperApplication","operatingSystem":"Web","description":"Convert HTML and Markdown to beautiful PDFs with a simple API call. Fast, reliable, developer-friendly.","offers":[{"@type":"Offer","price":"0","priceCurrency":"EUR","name":"Free","description":"100 PDFs/month"},{"@type":"Offer","price":"9","priceCurrency":"EUR","name":"Pro","description":"5,000 PDFs per month","billingIncrement":"P1M"}]}
 </script>
 <link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>⚡</text></svg>">
 <style>
@@ -404,7 +404,7 @@ html, body {
         <div class="price-amount">€9<span> /mo</span></div>
         <div class="price-desc">For production apps and businesses</div>
         <ul class="price-features">
-<li>2,500 PDFs per month</li>
+<li>5,000 PDFs per month</li>
           <li>All conversion endpoints</li>
           <li>All templates included</li>
           <li>Priority support</li>