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/lib/theme.contract.test.ts

@@ -48,6 +48,13 @@ describe("the media colours", () => {
   it("the player chrome's white text holds 4.5:1", () => {
     expect(hexContrast("#ffffff", media()["media-chrome"]!)).toBeGreaterThanOrEqual(4.5);
   });
+  // The amber that marks a place on the always-dark chrome and over a video — a playhead, a focused
+  // end's ring, the crop frame's ring — is the same in both themes: a light theme's warn is a dark
+  // amber, which fell under 3:1 on the clip strip (white 10% over the chrome). [WCAG 1.4.11]
+  it("a mark on the player chrome holds 3:1 on the clip strip", () => {
+    const strip = mixHex("#ffffff", media()["media-chrome"]!, 0.1);
+    expect(hexContrast(media()["media-mark"]!, strip)).toBeGreaterThanOrEqual(3);
+  });
 });
 
 it("styles exactly the schemes the app offers", () => {