[codex] Adjust schedule modal dismissal

[jjoonleo]

Summary

• Prevent schedule creation bottom sheets from closing when the backdrop/upper area is tapped.
• Allow Cupertino date/time/timer picker sheets to close from the backdrop.
• Move picker disposal callbacks to sheet completion so backdrop dismissal still triggers cleanup/validation.

Validation

• dart format --set-exit-if-changed lib/presentation/shared/components/cupertino_picker_modal.dart lib/presentation/calendar/screens/calendar_screen.dart lib/presentation/shared/components/bottom_nav_bar_scaffold.dart (formatted files, then commit amended)
• flutter analyze
• git diff --check HEAD~1 HEAD

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