feat: student mode support for thanks page (admin-auth only)

- add hidden student_mode field in add.php form
- pass mode=student through redirect to thanks.php in formulaire.php
- thanks.php renders clean student thank-you page (no header, centered button)
- add CSS for .thanks-student-page, .btn-new-form, .thanks-success, .thanks-error
- admin auth always required; student mode is purely UI variant on the physical machine

public/admin/actions/formulaire.php

@@ -9,6 +9,8 @@ ini_set('error_log', 'error.log');
 
 AdminAuth::requireLogin();
 
+$studentMode = isset($_POST['student_mode']) && $_POST['student_mode'] === '1';
+
 // Verify CSRF token
 if (!isset($_POST['csrf_token'], $_SESSION['csrf_token'])
     || !hash_equals($_SESSION['csrf_token'], $_POST['csrf_token'])) {
@@ -26,7 +28,11 @@ try {
 
     unset($_SESSION['csrf_token']);
 
-    header('Location: ../thanks.php?id=' . urlencode($thesisId));
+    $redirect = '../thanks.php?id=' . urlencode($thesisId);
+    if ($studentMode) {
+        $redirect .= '&mode=student';
+    }
+    header('Location: ' . $redirect);
     exit();
 
 } catch (Exception $e) {
@@ -35,11 +41,16 @@ try {
     App::flash('error', $e->getMessage());
     $_SESSION['form_data'] = $_POST;
 
+    $redirect = '../add.php';
+    if ($studentMode) {
+        $redirect .= '?mode=student';
+    }
+
     $autofocusField = ThesisCreateController::autofocusFieldForError($e->getMessage());
     if ($autofocusField !== null) {
         App::flashAutofocus($autofocusField);
     }
 
-    header('Location: ../add.php');
+    header('Location: ' . $redirect);
     exit();
 }