feat(admin): cancel buttons for in-flight scans + cancelled state UI

- per-row Cancel button in the scan-progress card; listeners are bound
  after the TS island renders its rows (module scope has no inline
  handlers), and the button is removed once its job turns terminal
- cancel-all icon button in the card header: resolves the server's
  active job list first, so it also covers scans this page never
  started (e.g. started before a refresh), then cancels each one
- polling treats cancelled as terminal, averages overall progress
  only over jobs that can still advance (so the bar can reach 100%),
  and the results panel reports cancelled scans instead of claiming
  "Scan Complete!"
- sidebar spinner resets on the scan_cancelled websocket event (same
  quiet handling as scan_error)
- setup wizard shows "Cancelled" as a terminal status in its poller

No new innerHTML assignment sites - rendering-ownership baseline
unchanged.

web/src/header.ts

@@ -92,6 +92,11 @@ const initializeScanListener = function (this: any) {
         this.scanning = false;
         this.scanProgress = 0;
         break;
+      case "scan_cancelled":
+        // A cancelled scan is not an error: stop the spinner quietly.
+        this.scanning = false;
+        this.scanProgress = 0;
+        break;
     }
   };