feat(queue): 404 on unknown hash, show error in UI

POST /api/queue/cancel answers 404 {"error":"hash not in queue"} for
untracked hashes instead of 200 ok:true. The UI surfaces the server's
error message instead of swallowing it.

ui/scripts.js

@@ -11,11 +11,18 @@ async function apiFetch(path, opts) {
       ...(opts && opts.headers),
     },
   });
+  const ct = res.headers.get('content-type') || '';
   if (!res.ok) {
     const text = await res.text().catch(() => '');
-    throw new Error(text || `HTTP ${res.status}`);
+    let msg = text || `HTTP ${res.status}`;
+    if (ct.includes('application/json')) {
+      try {
+        const data = JSON.parse(text);
+        if (data && data.error) msg = data.error;
+      } catch {} // Malformed JSON: keep the raw text
+    }
+    throw new Error(msg);
   }
-  const ct = res.headers.get('content-type') || '';
   if (ct.includes('application/json')) return res.json();
   return res.text();
 }
@@ -209,8 +216,11 @@ function renderQueue() {
             headers: { 'Content-Type': 'application/json' },
             body: JSON.stringify({ hash }),
           });
-        } catch {}
-        renderQueue();
+          renderQueue();
+        } catch {
+          btn.textContent = '[err]';
+          setTimeout(() => { btn.disabled = false; btn.textContent = '[x]'; }, 2000);
+        }
       });
     });
   }).catch(err => {