feat: website templating system — shared nav/footer partials, build-time HTML assembly

- Build script: scripts/build-pages.js (zero dependencies, Node.js only)
- Shared partials: nav.html, footer.html (single source of truth)
- 5 page templates in templates/pages/
- Output is byte-for-byte identical to current production
- Run: npm run build:pages
- Also fixes JSON-LD: 2,500 → 5,000 PDFs/month (was inconsistent)

package.json

@@ -4,8 +4,8 @@
   "description": "Markdown/HTML to PDF API with built-in invoice templates",
   "main": "dist/index.js",
   "scripts": {
-    "build:html": "node scripts/build-html.cjs",
-    "build": "tsc",
+    "build:pages": "node scripts/build-pages.js",
+    "build": "npm run build:pages && tsc",
     "start": "node dist/index.js",
     "dev": "tsx src/index.ts",
     "test": "vitest run"