Update Google Gemini preset to match Claude Official styling:
- Rename 'Google' to 'Google Official'
- Add GeminiIcon support in preset selector
- Add custom theme with Google blue (#4285F4) background
- Update PresetTheme type to support 'gemini' icon type
Changes:
- Add GeminiPresetTheme interface
- Add theme config to Google Official preset
- Import and render GeminiIcon in ProviderPresetSelector
- Update PresetTheme icon type to include 'gemini'
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