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/i18n/locales/en.json

@@ -179,8 +179,11 @@
     "claudeConfigDirDescription": "Override Claude configuration directory (settings.json) and keep claude.json (MCP) alongside it.",
     "codexConfigDir": "Codex Configuration Directory",
     "codexConfigDirDescription": "Override Codex configuration directory.",
+    "geminiConfigDir": "Gemini Configuration Directory",
+    "geminiConfigDirDescription": "Override Gemini configuration directory (.env).",
     "browsePlaceholderClaude": "e.g., /home/<your-username>/.claude",
     "browsePlaceholderCodex": "e.g., /home/<your-username>/.codex",
+    "browsePlaceholderGemini": "e.g., /home/<your-username>/.gemini",
     "browseDirectory": "Browse Directory",
     "resetDefault": "Reset to default directory (takes effect after saving)",
     "checkForUpdates": "Check for Updates",