fix(clip): marks on the player chrome keep their amber in a light theme, and a removed cut hands the focus on

- A media amber (`--media-mark`), the same in both themes, for what marks a place on the always-dark
  chrome or over a video: the clip strip's playhead and focused end, the crop frame's ring, Plex's
  intro / credits markers. A light theme's warn was a dark amber there, under 3:1 (contract-tested).
- The editor's remove-cut button rings inside itself (the track clipped its ring), and once it goes
  with its cut the focus moves to the cut beside it, or to Split here, not to <body>.

frontend/src/components/VideoEditor.partial.dom.test.tsx

@@ -147,10 +147,31 @@ describe("VideoEditor — a cut's remove button", () => {
     split();
     const remove = screen.getByRole("button", { name: "editor.removeCut" });
     expect(screen.getByRole("slider", { name: "editor.cutN" }).contains(remove)).toBe(false);
+    act(() => remove.focus());
     fireEvent.click(remove);
     expect(screen.queryByRole("slider", { name: "editor.cutN" })).toBeNull();
   });
 
+  it("hands the focus on to a cut beside it, or to Split here, never to <body>", async () => {
+    split();
+    const video = document.querySelector("video")!;
+    Object.defineProperty(video, "currentTime", { value: 70, configurable: true, writable: true });
+    fireEvent.timeUpdate(video);
+    fireEvent.click(screen.getByText("editor.splitHere"));
+    const [second] = screen.getAllByRole("button", { name: "editor.removeCut" }).slice(1);
+    act(() => second!.focus());
+    fireEvent.click(second!);
+    await waitFor(() =>
+      expect(document.activeElement).toBe(screen.getByRole("slider", { name: "editor.cutN" })),
+    );
+    const last = screen.getByRole("button", { name: "editor.removeCut" });
+    act(() => last.focus());
+    fireEvent.click(last);
+    await waitFor(() =>
+      expect(document.activeElement).toBe(screen.getByText("editor.splitHere").closest("button")),
+    );
+  });
+
   it("sits at the cut's foot, under the next segment's keep toggle at its head", () => {
     split();
     const remove = screen.getByRole("button", { name: "editor.removeCut" });