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/DataTable.tsx

@@ -4,7 +4,7 @@ import { ArrowDown, ArrowUp, ListFilter, X } from "lucide-react";
 import { useDismiss } from "../lib/useDismiss";
 import { useScrollFade } from "../lib/useScrollFade";
 import { sortRows, cycleSort, type SortState } from "../lib/columnSort";
-import Pager from "./Pager";
+import Pager, { hasPagerContent } from "./Pager";
 import { PageToolbar } from "./PageShell";
 
 // A reusable, client-side data table: per-column sort + filter (in the header) + pagination,
@@ -285,9 +285,8 @@ export default function DataTable<T>({
   // are header controls and belong only at the top.
   // Stays NULLABLE: the wrappers below decide whether to render a controls row at all, and an always
   // -truthy element would leave an empty `my-3` band (and an empty in-band PageToolbar) on a table
-  // with nothing to page.
-  const pagerNode =
-    totalPages > 1 || sorted.length > 0 ? (
+  // with nothing to page. The emptiness rule itself lives in Pager, so the two can't drift apart.
+  const pagerNode = hasPagerContent(sorted.length, size) ? (
       <Pager
         page={safePage}
         size={size}