fix(playback): show the Up next card near the real end of an episode

The card appeared a fixed 8 minutes before the end: most of a short
episode, and far into the story of a long one, with no way to hide it.

- Adaptive lead: 4% of the episode, clamped to 40 s … 3 min.
- A closing-credits chapter in the last third, when timeline segments
  carry one, brings the card forward to its start (capped at 5 min).
- Close button and Escape dismiss the card for the current next episode.
- After 10 s (not while hovered) the card collapses into a one-line pill.
- Seconds countdown in the last minute.
- New playback setting "Up next card" (default on) turns it off.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changes/playback-up-next-card-timing.md

@@ -0,0 +1,6 @@
+---
+type: fix
+area: playback
+---
+
+The "Up next" card now appears near the actual end of an episode — under a minute before the end of a short episode, about two minutes before the end of an hour-long one — and shrinks to a small pill after a few seconds. A close button or Escape hides it for the episode, and a playback setting turns it off.