fix(managers): address the E4 S3 code-review round 2 findings

- The portalled tag picker now FOLLOWS its trigger on scroll (rAF-coalesced)
  instead of closing. Closing on every wheel tick was a regression for the table,
  where the menu used to travel with its row; it still closes once the trigger
  leaves the viewport (nothing left to anchor to) or on resize.
- Place the menu by MEASUREMENT, not a px estimate: it opens below, then the
  layout effect flips it above only if the real box overflows. The old
  `rows * 30px` guess read short at font-scale 1.3 (the rows are rem), which
  could leave a fixed menu hanging off the bottom with no way to reach it.
- `toggle` only opens once a position exists, so the `open && coords` gate can
  never show a stale anchor.
- One owner for "is there anything to page": `hasPagerContent()` lives in Pager
  and DataTable calls it, instead of both re-deriving the rule — that drift is
  what left empty control bands behind in the first place.
- Reset the card page on the row COUNT too, not just the filter inputs, so an
  unsubscribe or a shrinking refetch can't leave a stale page to spring back.
- `parseCardSize` clamps to the sizes the pager actually offers (like clampView /
  clampStatus), so the select can't display one value while the grid pages by another.
- VirtualGrid's docblock now carries the stacking-context constraint: its rows are
  transformed, so anything floating an item renders must portal out.

frontend/src/components/Pager.tsx

@@ -6,6 +6,14 @@ import { ChevronLeft, ChevronRight } from "lucide-react";
 // (the channel manager's card layout) can paginate the same way. Controlled: the caller owns page +
 // size and slices its own rows; this only renders the controls and reports changes. `sizeLabel` lets
 // the caller say "Rows per page" or "Cards per page". Returns null when there's nothing to page.
+
+/** Whether a pager would render anything — the ONE owner of that rule. Callers that wrap the pager
+ * in their own chrome gate on this instead of re-deriving it: a drifted second copy is exactly what
+ * left empty control bands behind on tables with nothing to page. */
+export function hasPagerContent(total: number, size: number): boolean {
+  const totalPages = size <= 0 ? 1 : Math.max(1, Math.ceil(total / size));
+  return totalPages > 1 || total > 0;
+}
 export default function Pager({
   page,
   size,
@@ -52,7 +60,7 @@ export default function Pager({
     else setPageInput(String(safePage + 1));
   }
 
-  if (!(totalPages > 1 || total > 0)) return null;
+  if (!hasPagerContent(total, size)) return null;
   return (
     <div className="flex items-center gap-3 flex-wrap">
       {totalPages > 1 && (