fix(plex): address S2 round-2 review nits (typeahead highlight + tests)

- Reset `hi` on Escape and gate `activeOptionId` on `open`, so a dismissed
  panel never exposes an aria-activedescendant pointing at an unmounted option,
  and no stale highlight flashes on reopen (live-verified: aria-activedescendant
  is null when nothing is highlighted / the panel is closed).
- Strengthen the reopen test: setQ now writes back to the mock provider's q, so
  the test asserts a FRESH suggest query fires on retype (q="matrixx"), not just
  that a cached listbox re-appears.
- Add a combobox-ARIA test: role=combobox, aria-expanded toggles with the panel,
  and aria-controls associates the input with the listbox it opens.

Suite 171 green; typecheck + eslint clean.

frontend/src/components/PlexSearchBox.dom.test.tsx

@@ -20,7 +20,12 @@ vi.mock("../lib/useDebounced", () => ({ useDebounced: (v: unknown) => v }));
 
 // Controllable provider — PlexSearchBox reads q/scope/field and calls setQ/setField/setJumpTo.
 const plexState = { scope: "both", field: "all", q: "" } as { scope: string; field: string; q: string };
-const setQ = vi.fn();
+// setQ writes back to plexState.q so a typed value flows through to `dq` on the next render (the real
+// provider persists it) — lets the reopen test assert a fresh query fires on retype, not just that
+// the panel re-appears from cache.
+const setQ = vi.fn((v: string) => {
+  plexState.q = v;
+});
 const setField = vi.fn();
 const setJumpTo = vi.fn();
 vi.mock("./PlexProvider", () => ({
@@ -69,7 +74,7 @@ beforeEach(() => {
   plexState.scope = "both";
   plexState.field = "all";
   plexState.q = "";
-  setQ.mockReset();
+  setQ.mockClear(); // clear calls but KEEP the write-back implementation
   setField.mockReset();
   setJumpTo.mockReset();
   plexLibrary.mockReset();
@@ -161,9 +166,34 @@ describe("PlexSearchBox — typeahead", () => {
     fireEvent.keyDown(input, { key: "Escape" });
     await waitFor(() => expect(screen.queryByRole("listbox")).toBeNull());
 
-    // Typing again must re-open it without a blur+refocus round-trip.
+    // Typing again must re-open it without a blur+refocus round-trip AND fire a fresh suggest query
+    // for the new term (not just re-show the cached list).
+    plexLibrary.mockClear();
     fireEvent.change(input, { target: { value: "matrixx" } });
     await screen.findByRole("listbox");
-    expect(screen.getByRole("listbox")).toBeTruthy();
+    await waitFor(() =>
+      expect(plexLibrary).toHaveBeenCalledWith(expect.objectContaining({ q: "matrixx" })),
+    );
+  });
+
+  it("marks the input as a combobox and toggles aria-expanded with the panel", async () => {
+    plexState.q = "matrix";
+    plexLibrary.mockResolvedValue(result([card("m1", "The Matrix")]));
+    renderBox();
+    const input = screen.getByRole("combobox");
+    // Collapsed until focused.
+    expect(input.getAttribute("aria-expanded")).toBe("false");
+    expect(input.getAttribute("aria-controls")).toBeNull();
+
+    fireEvent.focus(input);
+    const list = await screen.findByRole("listbox");
+    // Expanded, and the input points at the listbox it controls.
+    expect(input.getAttribute("aria-expanded")).toBe("true");
+    expect(input.getAttribute("aria-controls")).toBe(list.id);
+
+    // Escape collapses it again and drops the association.
+    fireEvent.keyDown(input, { key: "Escape" });
+    await waitFor(() => expect(input.getAttribute("aria-expanded")).toBe("false"));
+    expect(input.getAttribute("aria-controls")).toBeNull();
   });
 });

frontend/src/components/PlexSearchBox.tsx

@@ -73,7 +73,9 @@ export default function PlexSearchBox() {
 
   const fields = availablePlexFields(scope);
   const pillLabel = t(plexFieldLabelKey(activeField));
-  const activeOptionId = hi >= 0 ? `${listId}-opt-${hi}` : undefined;
+  // Only reference an option while the listbox is actually open — a dangling aria-activedescendant
+  // on a collapsed combobox is an inconsistent a11y state.
+  const activeOptionId = open && hi >= 0 ? `${listId}-opt-${hi}` : undefined;
 
   return (
     <SearchBar
@@ -102,6 +104,7 @@ export default function PlexSearchBox() {
           pick(hi);
         } else if (e.key === "Escape") {
           setDismissed(true);
+          setHi(-1); // don't leave a stale highlight to flash on the next reopen
         }
       }}
       inputProps={{