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/main.py

@@ -6229,8 +6229,9 @@ def cmd_dashboard(args):
         print(f"Install them with:  {sys.executable} -m pip install 'fastapi' 'uvicorn[standard]'")
         sys.exit(1)
 
-    if not _build_web_ui(PROJECT_ROOT / "web", fatal=True):
-        sys.exit(1)
+    if "HERMES_WEB_DIST" not in os.environ:
+        if not _build_web_ui(PROJECT_ROOT / "web", fatal=True):
+            sys.exit(1)
 
     from hermes_cli.web_server import start_server