(if not already done) discard WEBUI install if node not installed

hatch_build.py

@@ -1,6 +1,7 @@
 """Custom hatch build hook to build the web frontend."""
 
 import os
+import shutil
 import subprocess
 import sys
 from pathlib import Path
@@ -15,20 +16,35 @@ class WebUIBuildHook(BuildHookInterface):
 
     def initialize(self, version: str, build_data: dict) -> None:
         """Run npm build to compile the frontend."""
-        # Skip on ReadTheDocs - npm is not available there
-        if os.environ.get("READTHEDOCS"):
-            return
-
         root = Path(self.root)
         app_dir = root / "app"
         output_dir = root / "src" / "experimaestro" / "webui" / "data"
 
+        def _handle_missing_webui():
+            """Create an empty directory to satisfy the force-include directive"""
+            # if the web UI is not built.
+            output_dir.mkdir(parents=True, exist_ok=True)
+
+        # Skip on ReadTheDocs - npm is not available there
+        if os.environ.get("READTHEDOCS"):
+            _handle_missing_webui()
+            return
+
         # Skip if output directory already exists and has content
         if output_dir.exists() and any(output_dir.iterdir()):
             return
 
         if not app_dir.exists():
             # No app directory - skip silently (might be an sdist without app)
+            _handle_missing_webui()
+            return
+
+        if not shutil.which("npm"):
+            print(
+                "warning: npm not found. Skipping web UI build.",
+                file=sys.stderr,
+            )
+            _handle_missing_webui()
             return
 
         print("Building web frontend...", file=sys.stderr)  # noqa: T201
@@ -41,8 +57,8 @@ def initialize(self, version: str, build_data: dict) -> None:
                 check=True,
                 capture_output=True,
             )
-        except subprocess.CalledProcessError as e:
-            print(f"npm install failed: {e.stderr.decode()}", file=sys.stderr)  # noqa: T201
+        except (subprocess.CalledProcessError, FileNotFoundError) as e:
+            print(f"npm install failed: {e!s}", file=sys.stderr)  # noqa: T201
             raise
 
         # Run npm build
@@ -56,8 +72,8 @@ def initialize(self, version: str, build_data: dict) -> None:
                 capture_output=True,
                 env=env,
             )
-        except subprocess.CalledProcessError as e:
-            print(f"npm build failed: {e.stderr.decode()}", file=sys.stderr)  # noqa: T201
+        except (subprocess.CalledProcessError, FileNotFoundError) as e:
+            print(f"npm build failed: {e!s}", file=sys.stderr)  # noqa: T201
             raise
 
         print("Web frontend built successfully.", file=sys.stderr)  # noqa: T201