feat(settings): add Gemini configuration directory support (#255)

* style: apply code formatting across backend and frontend

Apply cargo fmt and prettier formatting to improve code readability.
No functional changes.

Changes:
- Rust: multi-line assertion formatting (gemini_config, env_checker)
- Rust: simplify chained method calls (provider)
- TypeScript: add trailing commas to function parameters (codexProviderPresets)

* feat(settings): add Gemini configuration directory support

Add custom configuration directory support for Gemini:
- Add geminiConfigDir field to Settings type
- Extend DirectorySettings component with Gemini input
- Update useDirectorySettings hook for Gemini directory management
- Add i18n translations for Gemini directory settings

src/types.ts

@@ -97,6 +97,8 @@ export interface Settings {
   claudeConfigDir?: string;
   // 覆盖 Codex 配置目录（可选）
   codexConfigDir?: string;
+  // 覆盖 Gemini 配置目录（可选）
+  geminiConfigDir?: string;
   // 首选语言（可选，默认中文）
   language?: "en" | "zh";
   // Claude 自定义端点列表