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/css/admin.css

@@ -75,18 +75,6 @@
 /* ── Buttons ────────────────────────────────────────────────────────────── */
 .admin-form-footer {
     margin-top: var(--space-l);
-    padding-top: var(--space-m);
-}
-
-/* Sticky variant — pinned below admin header, top-right */
-.admin-form-footer--sticky {
-    position: sticky;
-    top: 0;
-    z-index: 10;
-    margin: 0 0 var(--space-m);
-    display: flex;
-    justify-content: flex-end;
-    gap: var(--space-s);
 }
 
 /* ── Admin button aliases — see common.css .btn base class ────────────── */

app/public/assets/css/common.css

@@ -427,6 +427,7 @@ main {
 .btn--primary {
     background: var(--accent-primary);
     color: var(--accent-foreground);
+    border: 1px solid transparent;
 }
 
 .btn--primary:hover {

app/public/assets/css/form.css

@@ -302,6 +302,10 @@
 /* ── Submit / form footer ───────────────────────────────────────────────── */
 .form-footer {
     margin-top: var(--space-l);
+    margin-bottom: var(--space-l);
+    display: flex;
+    gap: var(--space-s);
+    align-items: center;
 }
 
 .form-footer button {