Implement dynamic theme switching in Storybook and reset default theme to 'default'

[ryota-murakami]

📋 Task Description

Currently, Storybook stories with theme props are hardcoded to use the 'sunrise' theme. This prevents testing components with the default theme and other theme variations. We need to:

1. Reset default theme from 'sunrise' to 'default'
2. Implement theme switching addon/control in Storybook
3. Allow manual theme selection per story variant

🎯 Objectives

• Identify all stories with hardcoded theme props (e.g., BoardCard.stories.tsx)
• Change theme default from 'sunrise' to 'default'
• Implement Storybook theme switcher control
• Enable per-story theme override capability
• Document theme switching usage in stories
• Test all 14 available themes render correctly

📋 Current Themes Available

Light themes:

• default, sunrise, sandstone, mint, sky, lavender, rose

Dark themes:

• dark, midnight, graphite, forest, ocean, plum, rust

🛠️ Implementation Approach

1. Update mock data in stories: theme: 'sunrise' → theme: 'default'
2. Add theme arg to story args for manual override
3. Consider Storybook addon or global parameter for theme switching
4. Verify theme application across all component stories

📚 References

• Theme configuration: lib/theme.ts
• BoardCard story: components/Boards/BoardCard.stories.tsx:22
• Theme system: 14 predefined CSS themes in styles/themes/

✅ Acceptance Criteria

• Default theme is 'default', not 'sunrise'
• Users can switch themes in Storybook UI
• All theme variants render without errors
• Documentation updated for story authors