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/index.css

@@ -45,6 +45,7 @@
   --color-live: var(--live);
   --color-live-fg: var(--live-fg);
   --color-media-chrome: var(--media-chrome);
+  --color-media-mark: var(--media-mark);
 
   --font-sans: Inter, system-ui, -apple-system, Segoe UI, Roboto, sans-serif;
 
@@ -1154,6 +1155,9 @@ html[data-theme="light"] {
   --live-fg: #ffffff;
   --rating-media: #fcd34d; /* a score on a poster's black scrim */
   --media-chrome: #171717; /* the always-dark player's menus and bars, under white text */
+  /* A place marked on that chrome or over a video (a playhead, a focused end's ring, the crop
+     frame's ring, an intro / credits marker): amber in both themes, as the chrome is dark in both. */
+  --media-mark: #fbbf24;
 }
 
 /* Toast auto-dismiss countdown bar (shrinks left-to-right to zero) */