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.

flake.nix

@@ -104,6 +104,7 @@
             # The run.sh script will read from files at runtime
             envVars = [
               "PORT=${toString cfg.port}"
+              "HOST=${cfg.host}"
               "POSTGRES_SOCKET_PATH=${cfg.postgresSocketPath}"
             ] ++ [
               # API keys - if path, pass as-is; if string, pass directly
@@ -131,6 +132,12 @@
                 description = "Port to bind the backend server";
               };
 
+              host = mkOption {
+                type = types.str;
+                default = "0.0.0.0";
+                description = "Host address to bind the server (0.0.0.0 for all interfaces, 127.0.0.1 for localhost only)";
+              };
+
               postgresSocketPath = mkOption {
                 type = types.str;
                 default = "/run/postgresql";