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/i18n/locales/en/common.json

@@ -12,6 +12,8 @@
   "loading": "Loading…",
   "language": "Language",
   "somethingWrong": "Something went wrong.",
+  "itemCount_one": "{{formattedCount}} item",
+  "itemCount_other": "{{formattedCount}} items",
   "accessRequestsTitle": "Access requests",
   "accessRequestsMessage": "{{count}} pending — review them on the Users page.",
   "accessRequestsReview": "Review",

frontend/src/i18n/locales/hu/common.json

@@ -12,6 +12,8 @@
   "loading": "Betöltés…",
   "language": "Nyelv",
   "somethingWrong": "Hiba történt.",
+  "itemCount_one": "{{formattedCount}} elem",
+  "itemCount_other": "{{formattedCount}} elem",
   "accessRequestsTitle": "Hozzáférési kérések",
   "accessRequestsMessage": "{{count}} függőben — nézd át a Felhasználók oldalon.",
   "accessRequestsReview": "Áttekintés",