Implement Software Best Practices and Graceful API Key Management

- Integration of ruff, eslint, prettier for consistent code style
- Global error handling and structured logging in backend
- Comprehensive testing strategy with 100% pass rate (33 backend, 2 frontend)
- Graceful handling of missing Gemini API key (non-blocking startup)
- New /settings/status endpoint and UI warning indicators
- Full documentation update (ARCHITECTURE.md, CONTRIBUTING.md, README.md, GEMINI.md)
- Restored missing frontend logic and fixed UI syntax errors

.gitignore

@@ -3,11 +3,19 @@ __pycache__/
 *.py[cod]
 *$py.class
 venv/
-.env
-.env_garmin
+.venv/
 .coverage
 htmlcov/
 .pytest_cache/
+.ruff_cache/
+
+# Environment files
+.env
+.env_*
+!.env.example
+
+# Logs
+*.log
 
 # Node
 node_modules/
@@ -16,8 +24,9 @@ dist-ssr/
 *.local
 
 # Project specific
-data/local/*
-!data/local/.gitkeep
+backend/data/local/*
+!backend/data/local/.gitkeep
+backend/.garth/
 
 # IDEs
 .vscode/