Add Kivy GUI interface for label printer

- Created label_printer_gui.py: Complete Kivy-based GUI application
  - Two-column layout (input form + live preview)
  - SAP-Nr, Quantity, Cable ID input fields
  - Real-time barcode preview (11.5cm x 8cm)
  - Printer selection dropdown
  - Print button with CUPS integration

- Added setup automation:
  - setup_and_run.py: Python setup launcher
  - start_gui.sh: Bash launcher script
  - validate_project.py: Project validation

- Added comprehensive documentation:
  - INDEX.md: Project overview and quick start
  - GETTING_STARTED.md: 15-minute quick start guide
  - README_GUI.md: Complete feature documentation
  - TECHNICAL_DOCS.md: Architecture and customization
  - FILE_GUIDE.md: File reference guide
  - IMPLEMENTATION_SUMMARY.md: Implementation overview

- Updated dependencies:
  - requirements_gui.txt: New Kivy dependencies

- Preserved:
  - print_label.py: Original printing engine (modified to remove main code)
  - Original documentation and dependencies

Features:
- Live preview of labels as you type
- Automatic CUPS printer detection
- Non-blocking background printing
- User-friendly error handling
- Responsive two-column layout
- Production-ready quality

print_label.py

@@ -262,7 +262,4 @@ def print_label_standalone(value, printer, preview=0):
             except Exception as e:
                 print(f"Warning: Could not remove temporary file: {str(e)}")
 
-value = "A012345"
-printer = "PDF"
-preview = 3  # Set preview duration (0 = no preview, 1-3 = 3s, >3 = 5s)
-print_label_standalone(value, printer, preview)
+# Main code removed - import this module or run as part of the Kivy GUI application