feat(gemini): add Google Official branding with Gemini icon (#211)

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'

src/config/claudeProviderPresets.ts

@@ -14,8 +14,8 @@ export interface TemplateValueConfig {
  * 预设供应商的视觉主题配置
  */
 export interface PresetTheme {
-  /** 图标类型：'claude' | 'codex' | 'generic' */
-  icon?: "claude" | "codex" | "generic";
+  /** 图标类型：'claude' | 'codex' | 'gemini' | 'generic' */
+  icon?: "claude" | "codex" | "gemini" | "generic";
   /** 背景色（选中状态），支持 Tailwind 类名或 hex 颜色 */
   backgroundColor?: string;
   /** 文字色（选中状态），支持 Tailwind 类名或 hex 颜色 */