Toast action results and confirm destructive actions in one dialog

Inline banners and per-row status text reported action results all over
the app; they are replaced by a small toast stack (bottom-right, Level 3
floating well styling) that only speaks for actions: successes fade,
errors stay until dismissed, and form-field validation stays inline.

Destructive actions no longer use bespoke inline confirm steps (the
library detail's Confirm delete button) or fire immediately (duplicate
copies/items, delete page selections, temp cleanup, upload discard, job
cancellation): they all go through one promise-based confirm dialog
(confirmAction) with a danger variant, Escape/backdrop to cancel.

frontend/src/features/online/OnlineDetail.tsx

@@ -31,6 +31,7 @@ import { formatBytes, formatDate } from "@/lib/format";
 import type { DownloadTask, Paginated } from "@/lib/types";
 import { useAuth } from "@/store/auth";
 import { useE621 } from "@/store/e621";
+import { toast } from "@/store/toasts";
 
 const ratingText: Record<string, string> = {
   s: "text-ctp-green",
@@ -115,6 +116,7 @@ export function OnlineDetail({ postId }: { postId: number }) {
         queryKey: ["e621", "post", postId],
       });
     },
+    onError: (error) => toast.error(e621ErrorMessage(error)),
   });
 
   const [downloadTaskId, setDownloadTaskId] = useState<string | null>(null);
@@ -166,6 +168,7 @@ export function OnlineDetail({ postId }: { postId: number }) {
       setDownloadTaskId(task.task_id);
       void queryClient.invalidateQueries({ queryKey: ["status"] });
     },
+    onError: (error) => toast.error(errorMessage(error)),
   });
 
   const taskQuery = useQuery({
@@ -400,17 +403,6 @@ export function OnlineDetail({ postId }: { postId: number }) {
               ) : null}
             </p>
           ) : null}
-
-          {favoriteMutation.isError ? (
-            <p className="rounded-md border border-ctp-red/30 bg-ctp-red/10 px-3 py-2 text-sm text-ctp-red">
-              {e621ErrorMessage(favoriteMutation.error)}
-            </p>
-          ) : null}
-          {downloadMutation.isError ? (
-            <p className="rounded-md border border-ctp-red/30 bg-ctp-red/10 px-3 py-2 text-sm text-ctp-red">
-              {errorMessage(downloadMutation.error)}
-            </p>
-          ) : null}
         </>
       }
       media={