Add Internationalization (i18n) Support

[pbalduino]

Description

Add comprehensive internationalization support to make the app accessible for users worldwide, starting with English and Brazilian Portuguese.

Current Behavior

• All app strings are hardcoded in English
• No localization support
• No language selection in settings
• Interface language depends on code, not user preference

Requested Features

Core i18n Implementation

• Flutter intl package integration
• ARB (Application Resource Bundle) files for translations
• Automatic locale detection from device settings
• Fallback to English when locale not supported
• Support for right-to-left languages (future-ready)

Language Support

• English (en) - Default/fallback language
• Brazilian Portuguese (pt-BR) - Primary additional language
• Extensible architecture for future languages

Settings Integration

• Language selection option in settings screen
• Ability to override device language
• Immediate language switching without app restart
• Language preference persistence

Strings to Localize

• Screen Titles: "Guitar Tuner", "Settings", "Tunings"
• Tuning Names: "Standard Guitar", "Drop D", etc.
• Status Messages: "Listening...", "Perfectly In Tune", etc.
• Button Labels: String notes, navigation buttons
• Settings Labels: All setting options and descriptions
• Error Messages: Permission errors, audio issues
• Accessibility: All tooltip and screen reader text

Technical Implementation

Dependencies

dependencies:
  flutter_localizations:
    sdk: flutter
  intl: ^0.18.0
dev_dependencies:
  intl_utils: ^2.8.0

File Structure

lib/
├── l10n/
│   ├── app_en.arb          # English strings
│   ├── app_pt.arb          # Portuguese strings
│   └── l10n.dart           # Generated localizations
├── generated/
│   └── intl/               # Generated files
└── main.dart               # Locale configuration

Priority

🟡 Medium - Enhances accessibility for global users

Acceptance Criteria

• Flutter intl package is properly configured
• English (en) and Portuguese (pt-BR) translations are complete
• Device language is automatically detected
• Language can be changed in settings screen
• Language changes take effect immediately
• Fallback to English works for unsupported locales
• All user-facing strings are localized
• Musical terms are appropriately translated
• RTL support is architecturally ready
• Language preference persists between app restarts

Files Affected

• pubspec.yaml (dependencies)
• lib/main.dart (locale configuration)
• lib/l10n/ (new directory for translations)
• lib/screens/settings_screen.dart (language selector)
• lib/screens/tuner_screen.dart (localized strings)
• lib/screens/tunings_screen.dart (localized strings)
• lib/data/tunings.dart (localized tuning names)

Translation Considerations

Musical Terms

• Note names (C, D, E, F, G, A, B) remain in English internationally
• Tuning descriptions may need cultural adaptation
• "Flat" (♭) and "Sharp" (♯) symbols are universal

Cultural Adaptation

• Brazilian Portuguese: formal vs informal tone
• Date/time formats (not applicable to tuner)
• Number formats for frequency display

Future Extensibility

• Spanish (es) for Latin America
• French (fr) for France/Canada
• German (de) for Europe
• Japanese (ja) for Asia
• Support for region-specific variants (pt-PT vs pt-BR)

Task ID: I18N-001