[codex] Smooth default preparation edit transition

[jjoonleo]

Summary

• Keep the default preparation edit route on a stable Scaffold while loading.
• Fade from the loading state into the editable content with AnimatedSwitcher.
• Keep the preparation form bloc alive across spare-time updates so in-progress edits do not reset.
• Add a widget regression test for preserving edited preparation text after changing spare time.

Fixes #430

Validation

• flutter analyze
• flutter test test/presentation/my_page/preparation_spare_time_edit/preparation_spare_time_edit_screen_test.dart