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/e2e/channel.spec.ts

@@ -111,7 +111,7 @@ test.describe("channel view", () => {
   test("the channel chrome stays fixed while the video grid scrolls", async ({ page }) => {
     await openApp(page);
     await page.getByTestId("video-card-channel").first().click();
-    const count = page.getByTestId("feed-result-count");
+    const count = page.getByTestId("item-count");
     const title = page.getByTestId("channel-title");
     await expect(count).toBeVisible();
     await expect(page.getByTestId("video-card").first()).toBeVisible();

frontend/e2e/feed.spec.ts

@@ -8,7 +8,7 @@ test.describe("feed", () => {
     await openApp(page);
     await expect(page.getByTestId("video-card").first()).toBeVisible();
 
-    const count = page.getByTestId("feed-result-count");
+    const count = page.getByTestId("item-count");
     await expect(count).toHaveText(/\d/);
     const baseline = await count.innerText();
 
@@ -55,7 +55,7 @@ test.describe("feed", () => {
   // pins in the shell's toolbar band above the scroller, so it stays put while the cards scroll.
   test("the feed toolbar stays pinned while cards scroll", async ({ page }) => {
     await openApp(page);
-    const count = page.getByTestId("feed-result-count");
+    const count = page.getByTestId("item-count");
     await expect(count).toBeVisible();
     const topBefore = await count.evaluate((el) => el.getBoundingClientRect().top);