Add web dashboard build to Nix flake (#12194)

The web dashboard (Vite/React frontend) is now built as a separate Nix
derivation and baked into the Hermes package. The build output is
installed to a standard location and exposed via the `HERMES_WEB_DIST`
environment variable, allowing the dashboard command to use pre-built
assets when available (e.g., in packaged releases) instead of rebuilding
on every invocation.

hermes_cli/web_server.py

@@ -59,7 +59,7 @@ except ImportError:
         f"Install with: {sys.executable} -m pip install 'fastapi' 'uvicorn[standard]'"
     )
 
-WEB_DIST = Path(__file__).parent / "web_dist"
+WEB_DIST = Path(os.environ["HERMES_WEB_DIST"]) if "HERMES_WEB_DIST" in os.environ else Path(__file__).parent / "web_dist"
 _log = logging.getLogger(__name__)
 
 app = FastAPI(title="Hermes Agent", version=__version__)