refine: required confirmation_email field on both student forms, StudentEmail uses it directly

- Add dedicated 'confirmation_email' (type=email, required) field
  to student form at end of submission (partage + admin).
- ThesisCreateController now validates it is present and a valid
  email; form is rejected if missing/invalid.
- Autofocus mapping for confirmation_email errors.
- StudentEmail uses confirmation_email directly (removed extractEmail
  hack that mined email from free-form contact field).

app/public/admin/add.php

@@ -151,6 +151,12 @@ include APP_ROOT . '/templates/header.php';
             ?>
         </fieldset>
 
+        <!-- ═══════════════════ E-mail de confirmation ═══════════ -->
+        <fieldset>
+            <legend>E-mail de confirmation</legend>
+            <?php $name = 'confirmation_email'; $label = 'Adresse e-mail * :'; $value = old('confirmation_email'); $type = 'email'; $required = true; $placeholder = 'ton.email@exemple.be'; $hint = 'Nécessaire pour recevoir le récapitulatif de ta soumission.'; $attrs = withAutofocus('confirmation_email'); include APP_ROOT . '/templates/partials/form/text-field.php'; ?>
+        </fieldset>
+
         <div class="form-footer">
             <button type="submit" name="go">Soumettre</button>
         </div>