Add path validation, fix silent folder fallthrough and move traceback import in app.py

[agentksimha]

Changes made to ISSR_Communication_Analysis_Tool_Samuel_Kalu/app.py:

1. Added path validation in get_image_base64
   The function opened the file directly with no existence check. A missing asset throws a generic FileNotFoundError with no context about which asset was missing. Added an explicit check that raises FileNotFoundError with the exact path.

2. Fixed silent folder path fallthrough in process_multiple_videos
   When folder_path is provided but is not a valid directory, the original code silently falls through to the elif video_files branch with no feedback to the user. Added an explicit elif folder_path and not os.path.isdir(folder_path) branch that returns a clear error message to the user immediately.

3. Added logo asset validation in create_interface
   get_image_base64 was called with a hardcoded relative path with no existence check. If the assets folder is missing the entire interface crashes at startup. Added a check that logs a warning and renders the interface without the logo rather than crashing.

4. Moved traceback import to module level
   import traceback appeared twice inside except blocks. Importing inside exception handlers works but is bad practice and adds overhead every time the exception is triggered. Moved to the top of the file with the other imports.