feat: usar a cor escolhida tal e qual e opcao de suavizar

- TatuPalette.scheme aceita softenPrimary e passa a usar a seed como primary, onPrimary por contraste e surfaceTint, em vez da paleta tonal do Material 3
- nova opcao AppSettings.softenThemeColor (default false, ou seja cor exacta)
- interruptor 'Suavizar cor do tema' em Configuracoes > Aparencia, com l10n en/pt
- a opcao e sincronizada via account data (soften_theme_color) e mantida localmente como fallback, tal como a cor do tema
- setApplicationAccountColor e updateApplicationAccountConfig preservam o novo campo
- um unico sync de account data actualiza cor e opcao
- testes de serializacao e round-trip para o novo campo

lib/config/tatu_palette.dart

@@ -97,7 +97,14 @@ abstract class TatuPalette {
   /// When a [seed] is provided (e.g. a color chosen by the user or a system
   /// dynamic color), its tonal values are used for the primary family while
   /// the brand surfaces, accent and error colors are preserved.
-  static ColorScheme scheme(Brightness brightness, [Color? seed]) {
+  ///
+  /// Unless [softenPrimary] is set, the seed is used verbatim as the primary
+  /// color, because the Material 3 tonal palette noticeably dulls it down.
+  static ColorScheme scheme(
+    Brightness brightness, [
+    Color? seed,
+    bool softenPrimary = false,
+  ]) {
     final isDark = brightness == Brightness.dark;
     final p = isDark ? _dark : _light;
     final base = ColorScheme.fromSeed(
@@ -106,12 +113,17 @@ abstract class TatuPalette {
       dynamicSchemeVariant: DynamicSchemeVariant.rainbow,
     );
     final useBrandPrimary = seed == null;
+    // Only force the exact color when the caller did not ask for the soft
+    // Material 3 palette. `null` here means "keep the value from [base]".
+    final exactPrimary = !useBrandPrimary && !softenPrimary ? seed : null;
     return base.copyWith(
-      primary: useBrandPrimary ? p.primary : null,
-      onPrimary: useBrandPrimary ? p.onPrimary : null,
+      primary: useBrandPrimary ? p.primary : exactPrimary,
+      onPrimary: useBrandPrimary
+          ? p.onPrimary
+          : (exactPrimary == null ? null : _contrastOn(exactPrimary)),
       primaryContainer: useBrandPrimary ? p.primaryContainer : null,
       onPrimaryContainer: useBrandPrimary ? p.onPrimaryContainer : null,
-      surfaceTint: useBrandPrimary ? p.primary : null,
+      surfaceTint: useBrandPrimary ? p.primary : exactPrimary,
       secondary: p.secondary,
       onSecondary: p.onSecondary,
       secondaryContainer: p.secondaryContainer,
@@ -141,6 +153,12 @@ abstract class TatuPalette {
       surfaceBright: p.surfaceBright,
     );
   }
+
+  /// Picks the readable foreground for an arbitrary background color.
+  static Color _contrastOn(Color background) =>
+      ThemeData.estimateBrightnessForColor(background) == Brightness.dark
+      ? Colors.white
+      : Colors.black;
 }
 
 class _Scheme {
@@ -209,4 +227,4 @@ class _Scheme {
   final Color surfaceHighest;
   final Color surfaceDim;
   final Color surfaceBright;
-}
+}