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/PageShell.dom.test.tsx

@@ -124,4 +124,16 @@ describe("PageShell --rail-pinned-w / --content-mw", () => {
     render(<PageShell maxWidth="64rem">content</PageShell>);
     expect(root.style.getPropertyValue("--rail-pinned-w")).toBe("16.75rem");
   });
+
+  it("recomputes at runtime when --font-scale changes the root font, without looping", async () => {
+    setWidth(1650); // roomy at 16px/rem (1650 >= 1600) → overlay
+    render(<PageShell maxWidth="64rem">content</PageShell>);
+    expect(root.style.getPropertyValue("--rail-pinned-w")).toBe("2.25rem");
+    // The text-size slider changes the root font-size with NO resize event — the MutationObserver
+    // must recompute. At 17px/rem the threshold is 100rem*17 = 1700 > 1650, so the rail goes in-flow.
+    // (If the observer's self-write guard were broken this would loop forever and time out.)
+    root.style.fontSize = "17px";
+    await new Promise((r) => setTimeout(r, 0)); // flush the MutationObserver microtask
+    expect(root.style.getPropertyValue("--rail-pinned-w")).toBe("16.75rem");
+  });
 });