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/src/components/SortControl.tsx

@@ -67,7 +67,12 @@ export default function SortControl({
         type="button"
         ref={pop.refs.setReference}
         {...pop.getReferenceProps()}
-        aria-label={label}
+        // Carry the active sort key AND direction in the accessible name (the visible arrow is
+        // aria-hidden), matching ViewSwitcher — a screen reader announces "Sort: Date, Descending",
+        // not a bare "Sort".
+        aria-label={`${label}: ${summary}${
+          showArrow && active ? `, ${active.dir === "asc" ? ascLabel : descLabel}` : ""
+        }`}
         className={`glass-card glass-hover inline-flex items-center gap-1.5 rounded-full border px-3 py-1 text-sm transition ${
           open ? "border-accent" : "border-border"
         }`}