feat(sources): real-time file progress for sync and upload — background upload with success toast

frontend/sources.html

@@ -113,8 +113,14 @@
           to pull the latest and re-import.
         </p>
       </div>
-      <!-- #sync-result is the aria-live announcer for the last sync
-           result ("N added · …") — renders off the "bor:sync-status" event. -->
+      <!-- #sync-result is the aria-live announcer: the last sync
+           result ("N added · …") when a sync settles, and — phase 64 —
+           the LIVE file label while either job runs ("Syncing… <file>
+           (n/m)" / "Importing <file> (n/m)"), UNTRUNCATED (the button's
+           label span ellipsizes; screen readers hear the full
+           source/relative path, which also rides the button title).
+           After an upload settles it stays empty — the upload's counts
+           live on the Sources page (A3). -->
       <span class="sync-result" id="sync-result" role="status" aria-live="polite"></span>
       <!-- Sync failure banner — role="alert" so a failed sync is announced. -->
       <div class="kb-banner is-error" id="sync-error-banner" role="alert" hidden>