Refresh buttons turn from the press to the end of their scan; quick checks keep the count (#232, #233)

The buttons followed the feeds being checked, so they stopped between one feed and the next;
7ecbe82 held them for 600 ms over each gap. Now pressing one starts it, and scan_done stops
it only when it is the end of the scan asked for: scan_done carries `only` and `forced`, so a
scheduled scan finishing first does not stop a button whose scan is queued behind it. An
error event, a job that failed without a scan_done, stops them all.

A row's spinner stood in for its unread count from the moment its feed was checked; most
checks take milliseconds, so each count blinked as the scan went by. It now waits 300 ms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/dialogs.ts

@@ -729,7 +729,11 @@ function opmlModal(){
 }
 
 // No toast: the spinners on the rows being checked say it (issue #37).
-async function scanAll(){ await api('/api/fetch',{method:'POST',body:JSON.stringify({force:true})}); }
+async function scanAll(){
+  spinStart(null);
+  try{ await api('/api/fetch',{method:'POST',body:JSON.stringify({force:true})}); }
+  catch(e){ spinEnd(); throw e; }
+}
 $('#scanAll').onclick=scanAll;
 $('#prefs').onclick=prefsModal;
 // Someone the proxy signed in is signed out by the proxy: ipx's own sign-out cannot stick while