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/setup.ts

@@ -432,10 +432,15 @@ function initSetup(): void {
               running: `Scanning... ${Math.round((status.progress || 0) * 100)}%`,
               completed: `✓ Complete (${status.new_items || 0} items)`,
               failed: "✗ Failed",
+              cancelled: "✗ Cancelled",
             };
             job.statusText = statusMessages[status.status] || status.status;
 
-            if (status.status !== "completed" && status.status !== "failed") {
+            if (
+              status.status !== "completed" &&
+              status.status !== "failed" &&
+              status.status !== "cancelled"
+            ) {
               allComplete = false;
             }
           } else {