refactor(legacy): migrate theming to Material 3 + SfColors extension

Replace the ThemePort/ThemeCode abstraction (GetIt-registered adapter)
with a Riverpod-driven Material 3 ColorScheme, an SfColors ThemeExtension
for brand tokens, and a user-facing appearance selector for light/dark/
system modes. Persisted via SharedPreferences, reacts to system
brightness changes. Payments mode keeps the existing ThemePort API.

Highlights
- New legacy_theme package: LegacyAppTheme (light/dark), LegacyColorSchemes,
  SfColors ThemeExtension, LegacyThemePreferences, LegacyThemeNotifier,
  LegacyThemeSelector. Timeframe-based variants scaffolded but disabled.
- New /legacy/dashboard/control_panel/settings/appearance route + screen.
- MaterialApp.router picks the legacy theme only when isLegacyMode.
- ~90 ThemeCode.* usages migrated to colorScheme.* / context.sfColors.*.
- 25 widgets dropped the 'ThemePort theme' constructor param.
- ~145 hardcoded colors migrated (exact hex 1:1, grey.shade tiers,
  destructive red -> colorScheme.error, background whites -> surface).
  Content-over-color whites, transparents, and brand semantic reds/
  oranges/greens intentionally preserved.
- sf_localizations updated with appearance / appearanceDescription keys
  in all six locales.

pubspec.yaml

@@ -25,6 +25,7 @@ workspace:
   - modules/legacy/modules/settings
   - modules/legacy/packages/legacy_shared
   - modules/legacy/packages/legacy_device_state
+  - modules/legacy/packages/legacy_theme
   - packages/design_system
   - packages/flutter_treezor_entrust_sdk_bridge
   - packages/flutter_treezor_entrust_sdk_bridge/example