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)

public/admin/index.php

@@ -77,7 +77,7 @@ try {
 }
 ?>
 
-<?php require_once __DIR__ . "/inc/head.php"; ?>
+<?php require_once APP_ROOT . '/templates/admin/head.php'; ?>
 <script>
     function toggleAll(source) {
         const checkboxes = document.querySelectorAll('input[name="selected_theses[]"]');
@@ -285,4 +285,4 @@ try {
     <?php endif; ?>
 </main>
 
-<?php require_once __DIR__ . "/inc/footer.php"; ?>
+<?php require_once APP_ROOT . '/templates/admin/footer.php'; ?>