Break the two largest widget files into part/part-of libraries grouped by
concern, with no public API or behaviour change (private widgets keep working
because parts share the library namespace; all imports stay in the main file).
slide_preview.dart 4748 -> 426 lines + slides/previews/{text,bullets,
checklist,table,media,code,chart,overlays}.dart
app_shell.dart 1930 -> 996 lines + shell/{shell_actions,tab_bar,
welcome_screen,status_bar,shell_overlays}.dart
fullscreen_presenter.dart is intentionally left as-is: ~1.6k of its lines are a
single interactive _FullscreenPresenterState (38 setState calls), which a
mechanical split cannot reduce and extensions can't host (protected setState).
Shrinking it needs a behaviour-affecting sub-widget extraction, tracked
separately.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
