feat: add template delete — backend DELETE route, frontend trash button

- DELETE /templates/:id — backend route
- deleteTemplate() API client function
- Trash icon on each template list item (shows on hover)
- Confirms inline, no dialog needed
- Resets builder if the deleted template was being edited

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/lib/api.ts

@@ -169,6 +169,10 @@ export async function previewTemplate(data: {
   return request<TemplatePreview>("/templates/preview", { method: "POST", body: JSON.stringify(data) });
 }
 
+export async function deleteTemplate(id: string): Promise<{ data: { ok: boolean } | null; error: string | null }> {
+  return request<{ ok: boolean }>(`/templates/${id}`, { method: "DELETE" });
+}
+
 export async function getLifecycles(): Promise<{ data: Lifecycle[] | null; error: string | null }> {
   return request<Lifecycle[]>("/lifecycles");
 }