chore: add const to improve performance

[kinolaev]

see:

• https://dart-lang.github.io/linter/lints/prefer_const_constructors.html
• https://dart-lang.github.io/linter/lints/prefer_const_constructors_in_immutables.html
• https://dart-lang.github.io/linter/lints/use_key_in_widget_constructors.html

Also, CoffeeTimerApp has been moved to main.dart and MyApp has been removed.

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