feat: add new sound options

[leochiu-a]

Summary

This PR expands the sound effect options for the Elden Ring GitHub extension by adding two new iconic sound effects from the game. Users can now choose from four different celebration sounds when GitHub events occur (PR merge, create, approve, etc.), providing more personalization and variety to their GitHub workflow experience.

Key Changes

• Added Flask of Crimson Tears sound effect - the satisfying healing flask sound effect
• Added New Item Fanfare sound effect - the iconic item discovery chime
• Refactored sound type system to use a centralized constant array (SOUND_TYPES) for better maintainability
• Updated type definitions to use TypeScript const assertions for type-safe sound selection
• Enhanced popup UI with the two new sound options
• Updated tests to validate all four sound types
• Updated README documentation with new sound options and asset references

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• ♻️ Refactoring (no functional changes)
• 🎨 Style/formatting changes
• 🧪 Test improvements
• 🔧 Configuration changes

Test Plan

Manual Testing

• Load the extension in Chrome and navigate to any GitHub repository
• Click the extension icon to open settings
• Verify the sound type dropdown shows 4 options:
  • You Died
  • Lost Grace Discovered
  • Flask of Crimson Tears (new)
  • New Item Fanfare (new)
• Select "Flask of Crimson Tears" and click "Test Banner"
• Verify the Flask of Crimson Tears sound plays correctly
• Select "New Item Fanfare" and click "Test Banner"
• Verify the New Item Fanfare sound plays correctly
• Perform actual GitHub actions (create/merge/approve PR) and verify sounds play as expected

Automated Testing

• Run npm test to verify all unit tests pass
• Tests now validate that SOUND_TYPES contains all 4 expected sound types
• Tests verify the sound path construction works for all sound types

Breaking Changes

None

Checklist

• 📝 Code follows the style guidelines
• 👀 Self-review has been performed
• 🧪 Tests have been added/updated
• 📖 Documentation has been updated