flutter-widget-fundamentals

Applies core Flutter widget practice when writing or fixing widget code: stateless versus stateful choice, const widgets, the State lifecycle and disposing controllers, keys in reorderable lists, layout constraint errors, updating state without mutating shared lists, forms and validation, basic screen navigation, implicit animations, adaptive layout and reading error messages. Use when building or reviewing a widget or screen, fixing a layout or "unbounded height" error, a state that does not update or resets, a form, or when a newcomer needs the basics explained. Do not use for app layers and state management choice (flutter-architecture), theming (flutter-design-system), API calls (flutter-django-integration) or profiling.