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/partials/_nav_index.html

@@ -0,0 +1,10 @@
+<nav aria-label="Main navigation">
+  <div class="container">
+    <a href="/" class="logo">⚡ Doc<span>Fast</span></a>
+    <div class="nav-links">
+      <a href="#features">Features</a>
+      <a href="#pricing">Pricing</a>
+      <a href="/docs">Docs</a>
+    </div>
+  </div>
+</nav>