refactor: extract templates from public/

- Created /templates for main site (header.php, footer.php)
- Created /templates/admin for admin section (head.php, footer.php)
- Removed /public/includes and /public/admin/inc
- Updated all references in code and docs
- Tests passing ✅

Cleaner separation: /public only contains web-accessible files (PHP entry points + assets)

templates/admin/footer.php

@@ -0,0 +1,6 @@
+<footer>
+    <p>Formulaire fait avec ❤ en PHP et <a href="https://watercss.kognise.dev/">Water.css</a>.</p>
+</footer>
+</body>
+
+</html>