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/components/settings/SettingsDialog.tsx

@@ -220,6 +220,7 @@ export function SettingsDialog({
                       onResetAppConfig={resetAppConfigDir}
                       claudeDir={settings.claudeConfigDir}
                       codexDir={settings.codexConfigDir}
+                      geminiDir={settings.geminiConfigDir}
                       onDirectoryChange={updateDirectory}
                       onBrowseDirectory={browseDirectory}
                       onResetDirectory={resetDirectory}