Extract shared TFE form partial — single source of truth for add/edit/partage

Created templates/partials/form/form.php as the unified form template driven by
$mode ('add'|'edit'|'partage') and boolean flags for optional sections.

The three calling templates (templates/admin/add.php, templates/admin/edit.php,
partage/index.php renderShareLinkForm) now only set variables then include the
shared partial. ~200 lines of duplicated fieldset HTML eliminated.

app/public/assets/js/beforeunload-guard.js

@@ -0,0 +1,25 @@
+/**
+ * Beforeunload guard — prompts the user before navigating away from unsaved changes.
+ *
+ * Attach to any form with a data-beforeunload-guard attribute.
+ * No effect when JavaScript is unavailable (form posts normally).
+ */
+(function () {
+    var forms = document.querySelectorAll('form[data-beforeunload-guard]');
+    if (!forms.length) return;
+
+    var dirty = false;
+
+    for (var i = 0; i < forms.length; i++) {
+        var form = forms[i];
+        form.addEventListener('input', function () { dirty = true; });
+        form.addEventListener('change', function () { dirty = true; });
+        form.addEventListener('submit', function () { dirty = false; });
+    }
+
+    window.addEventListener('beforeunload', function (e) {
+        if (dirty) {
+            e.preventDefault();
+        }
+    });
+})();