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.go

@@ -183,7 +183,15 @@ func Admin(user User, stats AdminStats) templ.Component {
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 17, "Scanning Libraries</h3><button @click=\"hideScanProgress()\" class=\"icon-btn\" aria-label=\"Close\">")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 17, "Scanning Libraries</h3><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\">")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		templ_7745c5c3_Err = Icon("x-circle", "h-5 w-5").Render(ctx, templ_7745c5c3_Buffer)
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 18, "</button> <button @click=\"hideScanProgress()\" class=\"icon-btn\" aria-label=\"Close\">")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
@@ -191,7 +199,7 @@ func Admin(user User, stats AdminStats) templ.Component {
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 18, "</button></div><div class=\"mb-4\"><div class=\"flex justify-between text-sm mb-2\"><span style=\"color: var(--text-secondary)\">Overall Progress</span> <span id=\"scan-progress-text\" style=\"color: var(--text-primary)\">0%</span></div><div class=\"w-full rounded-full h-3\" style=\"background-color: var(--surface-hover);\"><div id=\"scan-progress-bar\" class=\"h-3 rounded-full transition-all duration-500\" style=\"width: 0%; background-color: var(--accent);\"></div></div><div id=\"scan-status\" class=\"text-sm mt-2\" style=\"color: var(--text-secondary)\">Starting scan...</div></div><div id=\"library-progress-list\" class=\"space-y-3\"></div><div id=\"scan-results\" class=\"hidden mt-6 p-4 rounded-xl border\" style=\"background-color: var(--bg-primary); border-color: var(--border);\"><h4 class=\"font-semibold mb-2 flex items-center gap-2\" style=\"color: var(--status-success);\">")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 19, "</button></div></div><div class=\"mb-4\"><div class=\"flex justify-between text-sm mb-2\"><span style=\"color: var(--text-secondary)\">Overall Progress</span> <span id=\"scan-progress-text\" style=\"color: var(--text-primary)\">0%</span></div><div class=\"w-full rounded-full h-3\" style=\"background-color: var(--surface-hover);\"><div id=\"scan-progress-bar\" class=\"h-3 rounded-full transition-all duration-500\" style=\"width: 0%; background-color: var(--accent);\"></div></div><div id=\"scan-status\" class=\"text-sm mt-2\" style=\"color: var(--text-secondary)\">Starting scan...</div></div><div id=\"library-progress-list\" class=\"space-y-3\"></div><div id=\"scan-results\" class=\"hidden mt-6 p-4 rounded-xl border\" style=\"background-color: var(--bg-primary); border-color: var(--border);\"><h4 class=\"font-semibold mb-2 flex items-center gap-2\" style=\"color: var(--status-success);\">")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
@@ -199,7 +207,7 @@ func Admin(user User, stats AdminStats) templ.Component {
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 19, "Scan Complete!</h4><div id=\"scan-results-content\" style=\"color: var(--text-secondary)\"></div><div class=\"mt-4 flex gap-2\"><button @click=\"window.location.reload()\" class=\"btn btn-primary\">Refresh to View Books</button> <button @click=\"hideScanProgress()\" class=\"btn btn-secondary\">Dismiss</button></div></div></div></div></main></body></html>")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 20, "Scan Complete!</h4><div id=\"scan-results-content\" style=\"color: var(--text-secondary)\"></div><div class=\"mt-4 flex gap-2\"><button @click=\"window.location.reload()\" class=\"btn btn-primary\">Refresh to View Books</button> <button @click=\"hideScanProgress()\" class=\"btn btn-secondary\">Dismiss</button></div></div></div></div></main></body></html>")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}