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/index.css

@@ -98,6 +98,12 @@
      place since App and Playlists both reference it. */
   --hdr-h: 4.25rem;
 
+  /* Width of the app scroller's scrollbar gutter (scrollbar-gutter: stable reserves it on the right).
+     Measured once at startup (App) and written here; the floating header and the toolbar band pad
+     their right edge by it so the fixed chrome lines up with the scrolled cards, not 15px past them.
+     Defaults to 0 for overlay-scrollbar platforms where the gutter is empty. */
+  --sbw: 0px;
+
   /* ===== Glass tunables — every value that materially drives the look lives here so it can be
      tuned in ONE place. ===== */
   /* GLOBAL baseline = "Adaptive": solid-enough dark glass that reads well on content-less chrome