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/DirectorySettings.tsx

@@ -14,6 +14,7 @@ interface DirectorySettingsProps {
   onResetAppConfig: () => Promise<void>;
   claudeDir?: string;
   codexDir?: string;
+  geminiDir?: string;
   onDirectoryChange: (app: AppId, value?: string) => void;
   onBrowseDirectory: (app: AppId) => Promise<void>;
   onResetDirectory: (app: AppId) => Promise<void>;
@@ -27,6 +28,7 @@ export function DirectorySettings({
   onResetAppConfig,
   claudeDir,
   codexDir,
+  geminiDir,
   onDirectoryChange,
   onBrowseDirectory,
   onResetDirectory,
@@ -104,6 +106,17 @@ export function DirectorySettings({
           onBrowse={() => onBrowseDirectory("codex")}
           onReset={() => onResetDirectory("codex")}
         />
+
+        <DirectoryInput
+          label={t("settings.geminiConfigDir")}
+          description={undefined}
+          value={geminiDir}
+          resolvedValue={resolvedDirs.gemini}
+          placeholder={t("settings.browsePlaceholderGemini")}
+          onChange={(val) => onDirectoryChange("gemini", val)}
+          onBrowse={() => onBrowseDirectory("gemini")}
+          onReset={() => onResetDirectory("gemini")}
+        />
       </section>
     </>
   );

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}