material-theming

# Theming Material 3 theming best practices for Flutter applications using `ThemeData` as the single source of truth for colors, typography, component styles, and spacing. ## Core Standards Apply these standards to ALL theming work: - **Use `ThemeData` as the single source of truth** — never inline colors or text styles in widgets - **Reference colors via `Theme.of(context).colorScheme`** — never `Colors.blue`, `Colors.red`, or any hardcoded `Color` values - **Reference text styles via `Theme.of(context).textTheme`** — never inline `TextStyle(...)` in widget code. `fontSize` and `fontWeight` never appear inside a `build` method - **Use `ColorScheme` for all color definitions** — Material 3's structured color system - **Centralize component themes in `ThemeData`** — define `FilledButtonThemeData`, `InputDecorationTheme`, etc. in the theme, not per-widget. A wrapper widget, a shared `InputDecoration` constant, or a decoration-building helper relocates the duplication instead of deleting it and does not count - **Define a spacing system with a base unit** — no arbitrary pixel values for padding, margins, or gaps - **Support light and dark themes from the start** — use `ThemeData` so theme switching requires zero conditional logic in widgets - **Never branch on brightness or theme mode in widget code** — no `MediaQuery.platformBrightnessOf`, no ternary on `Theme.of(context).brightness`, no `context.isDarkMode` extension. Two `ColorScheme` instances make the branch unnecessary...