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

Dockerfile

@@ -18,7 +18,9 @@ COPY package*.json ./
 RUN npm install --omit=dev
 
 COPY dist/ dist/
+COPY scripts/ scripts/
 COPY public/ public/
+RUN node scripts/build-html.cjs
 RUN rm -f public/swagger-ui && ln -s /app/node_modules/swagger-ui-dist public/swagger-ui
 
 # Create data directory and set ownership to docfast user