fix(managers): address the E4 S3 code-review round 3 findings

- The tag menu's "trigger is gone" test now measures against the SCROLLER, not the
  window. The page scroller starts ~230px down (measured), so a row that slid under
  the fixed toolbar was still "on screen" by window coords and the menu floated over
  the chrome, anchored to something invisible.
- Scroll-repositioning keeps whichever side the menu is anchored to instead of
  resetting it to "below" every frame — that reset made the layout effect flip it
  back, costing two renders and two forced layouts per frame near the bottom edge.
- One placement rule (`coordsFrom`) now serves the initial open, the layout
  correction and every reposition, so those can no longer drift apart.
- Card page: reset on the filter/sort INPUTS only, and separately COMMIT the clamp
  when the page falls out of range. Keying the reset on the row count (round 2) was
  too blunt — a background refetch that merely added a channel threw the reader off
  page 7 for something they didn't do.
- Pager derives `pageCount()` once and `hasPagerContent` reuses it, instead of the
  same expression twice in one file.
- Re-indent the pagerNode JSX left dangling by round 2's ternary edit.

frontend/src/components/DataTable.tsx

@@ -287,19 +287,19 @@ export default function DataTable<T>({
   // -truthy element would leave an empty `my-3` band (and an empty in-band PageToolbar) on a table
   // 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}
-        total={sorted.length}
-        pageSizeOptions={pageSizeOptions}
-        onPageChange={setPage}
-        onSizeChange={(s) => {
-          setSize(s);
-          setPage(0);
-        }}
-        sizeLabel={t("datatable.rowsPerPage")}
-      />
-    ) : null;
+    <Pager
+      page={safePage}
+      size={size}
+      total={sorted.length}
+      pageSizeOptions={pageSizeOptions}
+      onPageChange={setPage}
+      onSizeChange={(s) => {
+        setSize(s);
+        setPage(0);
+      }}
+      sizeLabel={t("datatable.rowsPerPage")}
+    />
+  ) : null;
 
   const controls =
     controlsLeading != null || pagerNode ? (