fix(frontend): source info popover tracks scroll

SourceRow's popover-position effect registered only a resize listener,
while the sibling popovers also recompute on capture-phase scroll. The
sidebar body scrolls (and the page scrolls on mobile), so an open
position-fixed popover detached from its row. Mirror the sibling
pattern.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/components/source-sidebar.tsx

@@ -544,7 +544,11 @@ function SourceRow({
     }
     recompute();
     window.addEventListener("resize", recompute);
-    return () => window.removeEventListener("resize", recompute);
+    window.addEventListener("scroll", recompute, true);
+    return () => {
+      window.removeEventListener("resize", recompute);
+      window.removeEventListener("scroll", recompute, true);
+    };
   }, [popoverOpen]);
 
   return (