Add host configuration for backend server and update documentation

- Introduced a new `host` option in `flake.nix` to specify the server's bind address, defaulting to `0.0.0.0`.
- Updated `run.sh` to use the `HOST` environment variable for server binding.
- Modified `config.py` to read the `HOST` from environment variables, defaulting to `0.0.0.0`.
- Enhanced `README.md` with instructions for setting up the `HOST` variable and clarified access details for development and production modes.
- Adjusted `vite.config.ts` to allow frontend access from other computers by default.

backend/app/core/config.py

@@ -9,7 +9,7 @@ class Settings(BaseSettings):
     
     # Server
     port: int = int(os.getenv("PORT", "8080"))
-    host: str = "127.0.0.1"
+    host: str = os.getenv("HOST", "0.0.0.0")  # Default to all interfaces
     
     # Database
     postgres_socket_path: str = os.getenv("POSTGRES_SOCKET_PATH", "/run/postgresql")