fix(a11y,test): deep-review round 2 — e2e helper attr, trigger names, observer test

Second gate round over the full branch surfaced 4 verified findings (1 was a
regression the round-1 role change introduced); a 5th (--sbw load jump) was
dropped as refuted (the header only mounts after auth+data, so --sbw is set
before it ever paints).

- e2e (medium): the round-1 switch of FilterSelect rows from role=option to
  menuitemcheckbox moved their state from aria-selected to aria-checked, but
  the Playwright helper expectContentType still asserted aria-selected, so the
  channel specs would fail whenever the e2e lane runs (not part of the fast
  gate, so it stayed green). Assert aria-checked.
- a11y (low): the FilterSelect and SortControl pill triggers set
  aria-label={label}, which overrides the visible "Label: value" text, so a
  screen reader announced only the group name, never the current selection.
  Carry the value (and, for sort, the direction) in the accessible name,
  matching ViewSwitcher. Updated the unit-test trigger name queries to match.
- tests (low): cover the PageShell font-scale MutationObserver recompute path
  (a post-mount root font-size change → --rail-pinned-w re-computes once, no
  loop) — previously only the pre-render and resize paths were exercised.

Verified live: the triggers now announce e.g. "Sort: Date, Descending" /
"Watch state: All". Gate green: tsc (app+node+e2e), 373 vitest, prettier,
eslint (0 err).

frontend/e2e/helpers.ts

@@ -36,12 +36,13 @@ export async function clickVisibleChannelLink(page: Page): Promise<boolean> {
 /**
  * Assert a content-type (e.g. "includeLive") is on/off via the Content-types FilterSelect dropdown.
  * The options only exist while the dropdown is open, so this opens it, checks the option's
- * `aria-selected`, then closes it (Escape) so the floating panel can't block a later click.
+ * `aria-checked` (the multi FilterSelect renders its rows as menuitemcheckbox), then closes it
+ * (Escape) so the floating panel can't block a later click.
  */
 export async function expectContentType(page: Page, value: string, on: boolean): Promise<void> {
   await page.getByTestId("feed-content").click();
   await expect(page.getByTestId(`feed-content-${value}`)).toHaveAttribute(
-    "aria-selected",
+    "aria-checked",
     on ? "true" : "false",
   );
   await page.keyboard.press("Escape");