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.

templates/admin.templ

@@ -97,9 +97,14 @@ templ Admin(user User, stats AdminStats) {
 								@Icon("refresh", "h-5 w-5")
 								Scanning Libraries
 							</h3>
-							<button @click="hideScanProgress()" class="icon-btn" aria-label="Close">
-								@Icon("close", "h-5 w-5")
-							</button>
+							<div class="flex items-center gap-2">
+								<button id="scan-cancel-all-btn" @click="cancelAllScans()" class="hidden icon-btn" aria-label="Cancel all scans" title="Cancel all scans">
+									@Icon("x-circle", "h-5 w-5")
+								</button>
+								<button @click="hideScanProgress()" class="icon-btn" aria-label="Close">
+									@Icon("close", "h-5 w-5")
+								</button>
+							</div>
 						</div>
 						<div class="mb-4">
 							<div class="flex justify-between text-sm mb-2">