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/assets/css/admin.css

@@ -1748,3 +1748,66 @@
 .form-footer button:hover {
     background: var(--accent-secondary);
 }
+
+/* ── Student thanks page ────────────────────────────────────────────────── */
+.thanks-student-page {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    min-height: 60vh;
+    text-align: center;
+}
+
+.thanks-success,
+.thanks-error {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    gap: var(--space-m);
+    max-width: 520px;
+}
+
+.thanks-success h1,
+.thanks-error h1 {
+    font-size: var(--step-3);
+    margin: 0;
+    letter-spacing: 0.06em;
+}
+
+.thanks-message {
+    font-size: var(--step-0);
+    color: var(--text-primary);
+    margin: 0;
+    line-height: 1.6;
+}
+
+.thanks-error p {
+    font-size: var(--step-0);
+    color: var(--text-secondary);
+    margin: 0;
+}
+
+.btn-new-form {
+    display: inline-flex;
+    align-items: center;
+    gap: var(--space-2xs);
+    padding: var(--space-m) var(--space-2xl);
+    background: var(--accent-primary);
+    color: var(--accent-foreground);
+    border: none;
+    border-radius: 6px;
+    font-size: var(--step-0);
+    font-weight: 600;
+    font-family: inherit;
+    text-decoration: none;
+    cursor: pointer;
+    letter-spacing: 0.04em;
+    transition: background 0.15s, transform 0.15s;
+    margin-top: var(--space-s);
+}
+
+.btn-new-form:hover {
+    background: var(--accent-secondary);
+    transform: translateY(-1px);
+}