feat(pickers): a list that failed to load offers Retry

A picker whose options failed to load said so, but the only way back was closing its dialog.
Combobox takes `onRetry`: a Retry beside the message for the pointer, and Enter in the field for
the keyboard (the panel is a listbox, which may hold options only, so the alert names Enter for
assistive tech). Every caller with an error state passes its query's refetch: both user pickers,
the channel picker and the two Plex pickers.

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

@@ -311,6 +311,34 @@ describe("Combobox · honest states", () => {
     expect(screen.queryByText("common.noResults")).toBeNull();
   });
 
+  // A failed list with no way back but closing its dialog was a dead end (E28 review, owner go).
+  it("offers a Retry on the error, by click and by Enter in the field, keeping the focus", () => {
+    const onRetry = vi.fn();
+    wrap(
+      <Combobox
+        label="User"
+        options={[]}
+        selected={[]}
+        onChange={vi.fn()}
+        error="Load failed"
+        onRetry={onRetry}
+      />,
+    );
+    const field = screen.getByRole("combobox");
+    field.focus();
+    fireEvent.focus(field);
+    // The listbox holds no button for assistive tech; the alert says how (Enter) instead.
+    expect(screen.getByRole("alert", { hidden: true }).textContent).toContain(
+      "common.retryWithEnter",
+    );
+    const retry = screen.getByText("common.retry");
+    expect(fireEvent.mouseDown(retry)).toBe(false); // the press does not take the focus
+    fireEvent.click(retry);
+    fireEvent.keyDown(field, { key: "Enter" });
+    expect(onRetry).toHaveBeenCalledTimes(2);
+    expect(document.activeElement).toBe(field);
+  });
+
   it("disabled: keeps the focus, opens nothing and picks nothing", () => {
     const onChange = vi.fn();
     wrap(<Combobox label="Genre" options={OPTIONS} selected={[]} onChange={onChange} disabled />);