Improve GUI and barcode generation

- Update GUI title to 'Label Printing' with white text
- Add character limit (25 chars) to all input fields
- Add number-only filter to quantity field
- Fix barcode generation in PDF module
- Create pdf_backup folder for storing generated PDFs
- Add pdf backup logging and confirmation
- Move demo files and tests to documentation folder
- Reorganize project structure for better clarity

documentation/how_to.txt

@@ -0,0 +1,11 @@
+install 
+
+sudo apt-get install libcups2-dev
+
+
+create venv or install with --breack-system-pakage
+
+
+python -m venv label 
+
+pip install -r requirements.txt