Fix Streamlit app path resolution for model and weight files

[sushantkhemalapure]

Summary

This PR improves reliability of the Streamlit app by fixing how model and weight file paths are resolved.

Problem

The app previously used hardcoded relative paths (e.g., ../../weights/...), which only work when executed from a specific working directory. Running the app from other locations caused missing file errors.

Solution

• Introduced APP_DIR to resolve paths relative to the app file location
• Added resolve_existing_path(...) helper that:
  • checks environment variable overrides
  • falls back to common local paths
  • raises clear errors if files are not found

Applied to

• CRAFT weights
• CRAFT refiner weights
• OCR model directory
• OCR processor directory

Impact

• No changes to OCR logic, UI, or preprocessing
• Improves portability and developer experience
• Makes app execution more robust across environments

Testing

• Verified app runs correctly from different working directories
• Verified fallback and error handling behavior