feat(toolbar): fixed-width ItemCount pill, lead the toolbar with it, align chrome to the cards

- New ItemCount: a generic, FIXED-WIDTH count pill (glass, non-interactive) — "192,050 items"
  (neutral noun, dynamic plural), min-w-[9rem] so a changing number never shifts the controls
  beside it; a million or more collapses to "1M+" with the exact figure in the title tooltip.
- Feed toolbar: ItemCount is the fixed leading element now; dropped the mx-1 separators so every
  gap is a uniform 1.5 (Content-types↔Source used to be wider than Watch-state↔Content-types).
- Chrome-to-card alignment (app-wide, via the shared Header + PageShell): the floating header and
  the toolbar band now span exactly the card grid's width — left-4, and a right edge of
  1rem + the scrollbar gutter (measured once into --sbw), so the fixed chrome lines up with the
  scrolled cards instead of running ~15px past them. Wraps cleanly to multiple rows on mobile.

Renames the count testid feed-result-count → item-count (E2Es updated).

frontend/src/App.tsx

@@ -136,6 +136,19 @@ export default function App() {
     if (meQuery.data && getActiveAccount() == null) setActiveAccount(meQuery.data.id);
   }, [meQuery.data?.id]);
 
+  // Measure the scrollbar's gutter width once and publish it as --sbw, so the floating header and the
+  // toolbar band can pad their right edge to line up with the scrolled cards (the app scroller reserves
+  // that gutter via scrollbar-gutter: stable). It's a device constant, so a one-shot mount measure is enough.
+  useEffect(() => {
+    const probe = document.createElement("div");
+    probe.style.cssText =
+      "position:absolute;top:-9999px;width:100px;height:100px;overflow:scroll;scrollbar-gutter:stable";
+    document.body.appendChild(probe);
+    const w = probe.offsetWidth - probe.clientWidth;
+    document.body.removeChild(probe);
+    document.documentElement.style.setProperty("--sbw", `${w}px`);
+  }, []);
+
   // Once signed in, drop any leftover ONE-TIME query params from the pre-login flow (?access, ?verify,
   // ?reset) — but NOT the nav/filter params, which the logged-in app now reads from and keeps in the
   // URL. Gated on auth so the logged-out Welcome page still reads its params first.