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/components/PageShell.tsx

@@ -103,7 +103,9 @@ export default function PageShell({
         className={`flex-1 min-w-0 min-h-0 flex flex-col ${header ? "pt-(--hdr-h)" : ""}`}
       >
         {banners}
-        <div ref={setToolbarSlot} className="shrink-0" />
+        {/* pr-[--sbw]: the toolbar band isn't the scroller, so without this it would run the
+            scrollbar-gutter's width past the scrolled cards on the right. Pad it back to card width. */}
+        <div ref={setToolbarSlot} className="shrink-0 pr-[var(--sbw)]" />
         <PageScroller scrollKey={scrollKey} fadeTop={fadeTop}>
           {children}
         </PageScroller>