fix: create parent directories for targets in subfolders

Build targets with subfolder paths (e.g. img/file.jpg) failed because
parent directories did not exist. Create them on demand in
_build_single_target before dispatching to providers.

Also clean up empty subdirectories in the clean command after removing
target files.

hokusai/builder.py

@@ -145,6 +145,9 @@ async def _build_single_target(
     provider_index: dict[str, Provider],
 ) -> None:
     """Build a single target by dispatching to the appropriate provider."""
+    # Ensure parent directories exist for targets in subfolders.
+    (project_dir / target_name).parent.mkdir(parents=True, exist_ok=True)
+
     target_cfg = config.targets[target_name]
 
     if isinstance(target_cfg, DownloadTargetConfig):