Playlist: auto-blend video under active scene effect; inter-track pause/intermission with continuing scene

- When a playlist video has an active scene effect, auto-apply 'screen' blend so
  the effect shows through (only if the user left blend at default 'normal';
  manual choices are respected). Restores on effect-off / next track.
- Per-track 'Pausa dopo' (seconds): after a track ends, keep its scene timeline
  running for N seconds as an intermission (scene elements placed past the song
  length appear during the pause), then advance. Countdown + badge in the row.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/style.css

@@ -401,6 +401,10 @@ textarea.textarea-lg {
 .scene-editor .cue-dim { flex: 0 0 auto; font-size: 10px; opacity: 0.55; }
 .scene-editor .se-foot { display: flex; gap: 6px; margin-top: 2px; flex-wrap: wrap; }
 .scene-editor .se-foot .se-clear { color: #ff9a9a; margin-left: auto; }
+.scene-editor .se-gap { display: flex; align-items: center; gap: 6px; margin-top: 9px; font-size: 12px; opacity: 0.9; }
+.scene-editor .se-gap input { width: 56px; text-align: center; padding: 6px 4px; }
+.scene-editor .se-gap small { opacity: 0.5; font-size: 10px; }
+.playlist .gap-badge { margin-left: 6px; font-size: 9.5px; padding: 1px 5px; border-radius: 6px; background: rgba(255,200,120,0.18); color: #ffd27a; vertical-align: middle; }
 
 /* ============ OUTPUT WINDOW ============ */
 body.output { cursor: none; background: #000; }