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/common.css

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