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/lib/channelLayout.ts

@@ -35,9 +35,13 @@ export function parseChannelLayout(raw: unknown): ChannelLayout {
 export const CARD_PAGE_SIZES = [10, 25, 50, 100];
 const CARD_SIZE_DEFAULT = 25;
 
-/** Coerce a stored cards-per-page value. Clamped like every other persisted value in this module:
- * a corrupt entry would otherwise make the page count NaN and slice the grid down to nothing, with
- * no in-app way back. */
+/** Coerce a stored cards-per-page value to one the pager actually OFFERS (the sizes above, or 0 =
+ * "All"). Clamped against the allowed set — like `clampView`/`clampStatus` — not merely type-checked:
+ * a corrupt entry would make the page count NaN and slice the grid down to nothing, and an
+ * off-menu number (a hand-edited 7, or an option a future build dropped) would page by a value the
+ * `<select>` cannot show, so the control and the behaviour would silently disagree. */
 export function parseCardSize(raw: unknown): number {
-  return typeof raw === "number" && Number.isFinite(raw) && raw >= 0 ? raw : CARD_SIZE_DEFAULT;
+  return typeof raw === "number" && (raw === 0 || CARD_PAGE_SIZES.includes(raw))
+    ? raw
+    : CARD_SIZE_DEFAULT;
 }