Validate local detect sources before inference

Add early local source validation to detect.py so missing local files and directories fail clearly before model setup. Preserve URL, webcam, screenshot, and matching glob behavior.\n\nThanks @Isha2790 for the contribution.

Author: Isha2790

detect.py

@@ -33,6 +33,7 @@
 import os
 import platform
 import sys
+from glob import glob, has_magic
 from pathlib import Path
 
 import torch
@@ -148,11 +149,18 @@ def run(
         ```
     """
     source = str(source)
-    save_img = not nosave and not source.endswith(".txt")  # save inference images
     is_file = Path(source).suffix[1:] in (IMG_FORMATS + VID_FORMATS)
     is_url = source.lower().startswith(("rtsp://", "rtmp://", "http://", "https://"))
     webcam = source.isnumeric() or source.endswith(".streams") or (is_url and not is_file)
     screenshot = source.lower().startswith("screen")
+
+    if not (webcam or screenshot or is_url) and not (
+        Path(source).exists() or (has_magic(source) and glob(source, recursive=True))
+    ):
+        raise FileNotFoundError(f"Source path '{source}' does not exist")
+
+    save_img = not nosave and not source.endswith(".txt")  # save inference images
+
     if is_url and is_file:
         source = check_file(source)  # download