Better argument handling of front-end-root (#7043)

* Better argument handling of front-end-root

Improves handling of front-end-root launch argument. Several instances where users have set it and ComfyUI launches as normal and completely disregards the launch arg which doesn't make sense. Better to indicate to user that something is incorrect.

* Removed unused import

There was no real reason to use "Optional" typing in ther front-end-root argument.

comfy/cli_args.py

 import argparse
 import enum
 import os
-from typing import Optional
 import comfy.options
 
 
@@ -166,13 +165,14 @@ parser.add_argument(
     """,
 )
 
-def is_valid_directory(path: Optional[str]) -> Optional[str]:
-    """Validate if the given path is a directory."""
-    if path is None:
-        return None
-
+def is_valid_directory(path: str) -> str:
+    """Validate if the given path is a directory, and check permissions."""
+    if not os.path.exists(path):
+        raise argparse.ArgumentTypeError(f"The path '{path}' does not exist.")
     if not os.path.isdir(path):
-        raise argparse.ArgumentTypeError(f"{path} is not a valid directory.")
+        raise argparse.ArgumentTypeError(f"'{path}' is not a directory.")
+    if not os.access(path, os.R_OK):
+        raise argparse.ArgumentTypeError(f"You do not have read permissions for '{path}'.")
     return path
 
 parser.add_argument(