feat(workflows): warn when disabling a built-in (reverts to legacy, not off)

Confirm dialog on the list page when toggling a built-in OFF, clarifying it
reverts to the previous built-in/legacy behaviour rather than turning the
automation off (review concern #4). The enable-refusal for unimplemented flows
surfaces via the existing toggle error toast (backend 409). EN + DE strings.

frontend/src/i18n/locales/de.json

@@ -212,6 +212,9 @@
     "enabled": "Aktiv",
     "disabled": "Inaktiv",
     "confirmDelete": "Diesen Workflow löschen?",
+    "toggle": {
+      "confirmDisableBuiltin": "Das Deaktivieren dieses eingebauten Ablaufs stellt das vorherige Standardverhalten wieder her – die Automatisierung wird dadurch nicht abgeschaltet. Fortfahren?"
+    },
     "test": {
       "title": "Testlauf",
       "hint": "Probelauf: durchläuft den ganzen Ablauf sofort (Wartezeiten übersprungen, Gates automatisch bestätigt), Nebeneffekte werden nur simuliert – keine echten E-Mails. Optional eine Entitäts-ID (z. B. eine Rechnung) angeben, damit Bedingungen sie lesen können.",

frontend/src/i18n/locales/en.json

@@ -212,6 +212,9 @@
     "enabled": "Enabled",
     "disabled": "Disabled",
     "confirmDelete": "Delete this workflow?",
+    "toggle": {
+      "confirmDisableBuiltin": "Disabling this built-in reverts to the previous built-in behaviour — it does not turn the automation off. Continue?"
+    },
     "test": {
       "title": "Test run",
       "hint": "Dry run: walks the whole flow now (waits skipped, gates auto-confirmed) with side effects mocked — no real emails. Optionally give an entity id (e.g. an invoice) so conditions can read it.",

frontend/src/pages/admin/workflows/WorkflowsListPage.tsx

@@ -73,6 +73,19 @@ export const WorkflowsListPage: React.FC = () => {
   const isEnabled = (w: WorkflowSummary) => w.enabled === true || w.enabled === 1;
   const isBuiltin = (w: WorkflowSummary) => w.is_builtin === true || w.is_builtin === 1;
 
+  // Disabling a built-in reverts to the legacy hardcoded behaviour (it does NOT
+  // stop the automation) — warn so the admin isn't surprised. Enabling is guarded
+  // server-side (a flow using unimplemented actions is refused with a clear error).
+  const toggle = (w: WorkflowSummary) => {
+    const next = !isEnabled(w);
+    if (!next && isBuiltin(w)) {
+      const msg = t('workflows.toggle.confirmDisableBuiltin',
+        'Disabling this built-in reverts to the previous built-in behaviour — it does not turn the automation off. Continue?') as string;
+      if (!window.confirm(msg)) return;
+    }
+    toggleMutation.mutate({ id: w.id, enabled: next });
+  };
+
   return (
     <div className="space-y-6">
       <div className="flex items-center justify-between gap-3 flex-wrap">
@@ -117,7 +130,7 @@ export const WorkflowsListPage: React.FC = () => {
                 </div>
                 <button
                   type="button"
-                  onClick={() => toggleMutation.mutate({ id: w.id, enabled: !isEnabled(w) })}
+                  onClick={() => toggle(w)}
                   className={`text-xs px-2 py-1 rounded-full border ${isEnabled(w)
                     ? 'bg-green-50 dark:bg-green-900/30 text-green-700 dark:text-green-300 border-green-300 dark:border-green-700'
                     : 'bg-neutral-50 dark:bg-neutral-800 text-neutral-500 dark:text-neutral-400 border-neutral-300 dark:border-neutral-600'}`}