feat(player): clip mode - mark a range of what is playing and cut it into a download

Both players get a scissors in their control row (Plex: the control row; YouTube: the actions and
the docked bar), `c`, and `[` / `]` to mark at the playhead. The clip panel lives in the player, so
the watched video is the preview: an overview of the whole item and a zoomed strip around the
selection whose ends are sliders (arrows step a tenth, Shift a second), Start / End fields with
nudges and "to the playhead", the length, a looping Play selection, a name, Cut. Escape closes the
panel before the player and the focus goes back to the scissors. A cut is a toast with a Downloads
link plus the usual Ready notification.

- ui/RangeTrack + RulerTicks: the shared, domain-free time track (regions, keyboard-steppable
  handles with a 1.5rem hit area, a playhead, a background slot, over a window of the item). The
  Downloads editor's cuts moved onto it, so they are keyboard-steppable now.
- lib/clipRange holds the marking rules (1 s to 10 min, the server's), lib/timecode the editing
  timecode the editor's private tc/parseTc became.
- keymap: a symbol typed through AltGr answers its bare-symbol row - on the HU layout `[` is
  AltGr+F, which arrives as Ctrl+Alt; letter and Mod rows still never match Ctrl+Alt.
- The YouTube stage sits in a wrapper with the panel, so the maximised stage's blur escape matches
  at any depth.
- media-scope: fields on the always-dark player chrome read white in a light theme too.

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

@@ -101,3 +101,26 @@ describe("VideoEditor — a split export that fails part-way", () => {
     expect(enqueue.mock.calls[2]![0].display_name).toBe("Talk (2/2)");
   });
 });
+
+// The cuts sit on the shared time track: each is a slider the keyboard steps, and a step stops a
+// tenth short of the next cut, so no segment ever closes up.
+describe("VideoEditor — a cut on the keyboard", () => {
+  it("steps a focused cut with the arrows and stops it before its neighbours", () => {
+    vi.spyOn(api, "downloadStoryboard").mockResolvedValue({ available: false } as never);
+    render(
+      <QueryClientProvider client={new QueryClient()}>
+        <VideoEditor job={job} onClose={() => {}} />
+      </QueryClientProvider>,
+    );
+    const video = document.querySelector("video")!;
+    Object.defineProperty(video, "currentTime", { value: 50, configurable: true, writable: true });
+    fireEvent.timeUpdate(video);
+    fireEvent.click(screen.getByText("editor.splitHere"));
+    const cut = screen.getByRole("slider", { name: "editor.cutN" });
+    expect(cut.getAttribute("aria-valuenow")).toBe("50");
+    fireEvent.keyDown(cut, { key: "ArrowRight", shiftKey: true });
+    expect(cut.getAttribute("aria-valuenow")).toBe("51");
+    fireEvent.keyDown(cut, { key: "End" });
+    expect(cut.getAttribute("aria-valuenow")).toBe("99.9");
+  });
+});