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/PlexPlayer.controls.dom.test.tsx

@@ -927,3 +927,60 @@ describe("the fullscreen button", () => {
     delete (HTMLElement.prototype as { requestFullscreen?: unknown }).requestFullscreen;
   });
 });
+
+// Clip mode: `[` / `]` mark at the playhead (opening the panel), the panel takes the control row's
+// place, Escape closes the panel before it would close the player, and the scissors gets the focus
+// back. Cut sends this item and the audio track that is playing.
+describe("PlexPlayer — clip mode", () => {
+  const key = (k: string, init: KeyboardEventInit = {}) =>
+    act(
+      () =>
+        void window.dispatchEvent(
+          new KeyboardEvent("keydown", { key: k, bubbles: true, cancelable: true, ...init }),
+        ),
+    );
+
+  it("opens on [ with Start at the playhead, and Escape closes only the panel", async () => {
+    mount({ details: { a: item("a") } });
+    await playingAt("/media/a", 120);
+    expect(screen.queryByTestId("clip-panel")).toBeNull();
+    key("[");
+    expect(screen.getByTestId("clip-panel")).toBeTruthy();
+    expect(screen.getByRole("slider", { name: "clip.in" }).getAttribute("aria-valuenow")).toBe(
+      "120",
+    );
+    // The seek bar and the control row make room for it.
+    expect(screen.queryByLabelText("plex.player.playPause")).toBeNull();
+    key("Escape");
+    expect(screen.queryByTestId("clip-panel")).toBeNull();
+    expect(screen.getByLabelText("plex.player.playPause")).toBeTruthy();
+    await waitFor(() =>
+      expect(document.activeElement).toBe(screen.getByRole("button", { name: "clip.open" })),
+    );
+  });
+
+  it("marks End on ] typed through AltGr (the HU layout's ])", async () => {
+    mount({ details: { a: item("a") } });
+    await playingAt("/media/a", 200);
+    key("]", { ctrlKey: true, altKey: true, modifierAltGraph: true } as KeyboardEventInit);
+    expect(screen.getByRole("slider", { name: "clip.out" }).getAttribute("aria-valuenow")).toBe(
+      "200",
+    );
+  });
+
+  it("cuts this item with the audio track that is playing", async () => {
+    const enqueue = vi.spyOn(api, "enqueueClip").mockResolvedValue({ id: 9 } as never);
+    mount({ details: { a: item("a") } });
+    await playingAt("/media/a", 60);
+    fireEvent.click(screen.getByRole("button", { name: "clip.open" }));
+    fireEvent.click(screen.getByRole("button", { name: "clip.cut" }));
+    await waitFor(() => expect(enqueue).toHaveBeenCalled());
+    expect(enqueue.mock.calls[0]![0]).toMatchObject({
+      source: "plex",
+      ref: "a",
+      start_s: 60,
+      end_s: 90,
+      audio_track: 0,
+    });
+  });
+});