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

start_gui.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+
+# Label Printer GUI - Quick Start Script
+# This script sets up and runs the Label Printer GUI application
+
+set -e
+
+echo "=========================================="
+echo "Label Printer GUI - Setup & Run"
+echo "=========================================="
+echo ""
+
+# Check Python installation
+echo "[1/4] Checking Python installation..."
+if ! command -v python3 &> /dev/null; then
+    echo "❌ Python 3 not found. Please install Python 3.7 or higher."
+    exit 1
+fi
+PYTHON_VERSION=$(python3 --version | cut -d' ' -f2)
+echo "✓ Python $PYTHON_VERSION found"
+echo ""
+
+# Check CUPS
+echo "[2/4] Checking CUPS (printer service)..."
+if ! command -v lpstat &> /dev/null; then
+    echo "⚠ CUPS not found. Please install with: sudo apt-get install cups"
+    echo "  Proceeding anyway - will use PDF printer"
+else
+    echo "✓ CUPS found"
+    echo "  Available printers:"
+    lpstat -p -d | head -5
+fi
+echo ""
+
+# Install dependencies
+echo "[3/4] Installing Python dependencies..."
+if [ -f "requirements_gui.txt" ]; then
+    pip install -r requirements_gui.txt
+    echo "✓ Dependencies installed"
+else
+    echo "⚠ requirements_gui.txt not found"
+    echo "  Installing Kivy and related packages manually..."
+    pip install kivy python-barcode pillow pycups
+fi
+echo ""
+
+# Run the application
+echo "[4/4] Starting Label Printer GUI..."
+echo "=========================================="
+echo ""
+python3 label_printer_gui.py
+