Add copyright information page with navigation

[Copilot]

Adds a dedicated copyright information page accessible from the main roulette screen.

Changes

New Copyright Screen

• lib/screens/copyright_screen.dart: Full-screen copyright page with app info, license details, and Flutter's built-in license viewer integration
• Routing: Extended main.dart to handle /copyright route alongside existing / route

Navigation Updates

• Main Screen: Added copyright button alongside existing "Copy URL" button in horizontal layout
• Back Navigation: Copyright screen includes proper app bar with back button

Internationalization

• English/Dutch: Added copyright-related strings to intl_en.arb and intl_nl.arb
• Localized Content: Copyright text, button labels, and page titles support both languages

Usage

Users click the "Copyright" button on the main screen to view application copyright information and access third-party licenses via the integrated license viewer.

// New route handling in main.dart
onGenerateRoute: (settings) {
  switch (settings.name) {
    case '/copyright':
      return CustomPageRoute(builder: (context) => const CopyrightScreen(), settings: settings);
    case '/':
    default:
      return CustomPageRoute(builder: (context) => const RouletteScreen(), settings: settings);
  }
},

Test Coverage

• test/copyright_screen_test.dart: Copyright page display and localization
• test/navigation_test.dart: Route navigation between screens

Original prompt

Please add a new page that shows copyright information

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