feat: make API URL configurable and improve code quality

- Add configurable API URL support via environment variables
- Update nginx configuration with environment variable substitution
- Create nginx template for dynamic proxy configuration
- Update Docker configuration for environment variable handling
- Fix hardcoded localhost:8000 references in error messages
- Add proper TypeScript types for health check API
- Format all code with Prettier for consistency
- Update documentation with configuration instructions
- Improve error messages to be environment-agnostic
- Fix duplicate imports and type safety issues

BREAKING: API URL is now configurable via VITE_API_URL (dev) and API_BACKEND_URL (prod)

leggend/main.py

@@ -66,7 +66,8 @@ def create_app() -> FastAPI:
         allow_origins=[
             "http://localhost:3000",
             "http://localhost:5173",
-        ],  # SvelteKit dev servers
+            "http://frontend:80",
+        ],  # Frontend container and dev servers
         allow_credentials=True,
         allow_methods=["*"],
         allow_headers=["*"],