revert: the Modal Escape guard for a fullscreen own-content video

Chrome does not hand the page the Escape it spends on leaving fullscreen (checked with a real
keypress: the first Esc leaves fullscreen, the second closes the player). The guard defended a case
that does not happen, under a comment saying it does. This reverts 121ebeeb.

frontend/src/components/ui/Modal.fullscreen.dom.test.tsx

@@ -106,39 +106,3 @@ describe("a dialog whose own content goes fullscreen", () => {
     expect(video.closest("body > div")).toBe(host);
   });
 });
-
-describe("Escape while the dialog's own content is fullscreen", () => {
-  const escape = () =>
-    act(() => {
-      window.dispatchEvent(new KeyboardEvent("keydown", { key: "Escape", bubbles: true }));
-    });
-
-  it("only leaves fullscreen (the browser's part) — the dialog stays open", () => {
-    // The browser spends that Escape on leaving fullscreen and still hands it to the page, with the
-    // video still the fullscreen element: the dialog closed with it, and the viewer lost the player.
-    const onClose = vi.fn();
-    render(
-      <Modal initialFocus="close" title="Player" onClose={onClose}>
-        <video aria-label="film" />
-      </Modal>,
-    );
-    setFullscreen(screen.getByLabelText("film"));
-    escape();
-    expect(onClose).not.toHaveBeenCalled();
-  });
-
-  it("still closes a dialog opened over something else that is fullscreen", () => {
-    const stage = document.createElement("div");
-    document.body.appendChild(stage);
-    setFullscreen(stage);
-    const onClose = vi.fn();
-    render(
-      <Modal initialFocus="close" title="Download" onClose={onClose}>
-        <p>form</p>
-      </Modal>,
-    );
-    escape();
-    expect(onClose).toHaveBeenCalledTimes(1);
-    stage.remove();
-  });
-});