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/import.php

@@ -277,7 +277,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['csv_file'])) {
     $_SESSION['csrf_token'] = bin2hex(random_bytes(32));
 }
 ?>
-<?php require_once __DIR__ . '/inc/head.php'; ?>
+<?php require_once APP_ROOT . '/templates/admin/head.php'; ?>
 
     <main>
     <h2>Importer des TFE depuis un fichier CSV</h2>
@@ -347,4 +347,4 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['csv_file'])) {
     <p>Voir: <code>../db/Database_TFE_test.csv</code></p>
 </main>
 
-<?php require_once __DIR__ . "/inc/footer.php"; ?>
+<?php require_once APP_ROOT . '/templates/admin/footer.php'; ?>