- apply_theme now re-renders already-drawn chat bubbles in the new palette:
  ChatPanel.refresh_content() rebuilds each visible item's rich renderable (the
  item renderers read get_active_theme(), already switched) and drops the
  per-widget Content/height caches so the next paint rebuilds instead of
  returning the old-theme cached Content. Theme switch / live preview no longer
  leaves the conversation in the previous palette (5.B1).
- theme_picker live preview is debounced (~100 ms coalesce via set_timer):
  apply_theme now re-renders every bubble, so applying it on every highlight
  while scrolling the theme list would stutter — rapid moves collapse into one
  apply, with the freshest theme winning. Escape/cancel dismiss the pending
  timer so it cannot fire after a restore and re-apply the highlighted theme
  over the restored one (4.P2).
- ChatModel.items is now bounded: a cap (visible window * 3) trims the oldest
  items off the front via a single _append/_trim path, so a long autonomous
  session no longer grows items without limit. The visible window is much
  smaller than the cap, so trimming only drops already-off-screen items and the
  truncation hint still reads correctly (3.P3).
- stream_end purges empty assistant/thinking bubbles off the tail (while-pop)
  instead of a full-history list-comprehension copy — empty bubbles are created
  at the end of the turn, so O(tail) suffices (1.P1).

Tests: apply_theme re-renders bubbles; theme picker debounces + escape cancels;
chat_model cap trims front / cap=None keeps everything; stream_end keeps
non-empty assistant + purges trailing empty thinking. Full suite: 946 passed, 1 skipped.