fix(ui): a picker whose list failed says so, and takes no pick while one is being written

- Combobox gains `error` (said in the list, not "No matches") and a soft `disabled` (keeps the
  focus, announced, opens and changes nothing).
- UserPicker passes both: the share dialog's recipient picker is off while a share is pending, and
  the share and quota pickers say when the user list failed to load.
- ChannelPicker and the Plex playlist/collection pickers say a failed load too.
- The channel menu's Tags row says loading / failed instead of "no tags yet".

frontend/src/components/ui/Combobox.dom.test.tsx

@@ -276,6 +276,33 @@ describe("Combobox", () => {
 // PageUp/PageDown (E11, owner request): the sidebar's channel filter and the Plex pickers run to
 // hundreds of rows, and floating-ui's list navigation has only the arrows. jsdom has no layout, so a
 // page is the fixed fallback of 10 rows; the measured page is pinned in lib/listPaging.
+// E28 review J1 + J2: a failed list said "No matches" (an error dressed as an empty result), and a
+// picker whose pick was still being written took another pick.
+describe("Combobox · honest states", () => {
+  it("says the list failed to load, not that nothing matched", () => {
+    wrap(
+      <Combobox label="User" options={[]} selected={[]} onChange={vi.fn()} error="Load failed" />,
+    );
+    fireEvent.focus(screen.getByRole("combobox"));
+    expect(screen.getByText("Load failed")).toBeTruthy();
+    expect(screen.queryByText("common.noResults")).toBeNull();
+  });
+
+  it("disabled: keeps the focus, opens nothing and picks nothing", () => {
+    const onChange = vi.fn();
+    wrap(<Combobox label="Genre" options={OPTIONS} selected={[]} onChange={onChange} disabled />);
+    const field = screen.getByRole("combobox");
+    field.focus();
+    fireEvent.focus(field);
+    expect(screen.queryByRole("option", { hidden: true })).toBeNull();
+    fireEvent.keyDown(field, { key: "ArrowDown" });
+    fireEvent.keyDown(field, { key: "Enter" });
+    expect(onChange).not.toHaveBeenCalled();
+    expect(field.getAttribute("aria-disabled")).toBe("true");
+    expect(document.activeElement).toBe(field); // not natively disabled: the focus stays put
+  });
+});
+
 describe("Combobox · paging", () => {
   const rows = (disabled: number[] = []) =>
     Array.from({ length: 25 }, (_, i) => ({ value: `Row ${i}`, disabled: disabled.includes(i) }));