Add Windows support with cross-platform printer detection

- Make app cross-platform compatible (Windows, Linux, macOS)
- Replace CUPS-only printer detection with platform-aware code
- Add Windows printer support using win32print/win32api
- Add macOS printer support using lpstat
- Keep Linux CUPS support
- Add requirements_windows.txt for Windows installation
- Add comprehensive WINDOWS_SETUP.md guide
- Fallback to PDF output on all platforms
- Tested on Linux, ready for Windows/macOS

WINDOWS_SETUP.md

@@ -0,0 +1,114 @@
+# Label Printer GUI - Windows Setup Guide
+
+## Installation Steps
+
+### 1. Install Python
+- Download Python 3.11+ from [python.org](https://www.python.org/downloads/)
+- **Important**: Check "Add Python to PATH" during installation
+
+### 2. Create Virtual Environment
+```bash
+python -m venv venv
+venv\Scripts\activate
+```
+
+### 3. Install Dependencies
+```bash
+pip install -r requirements_windows.txt
+```
+
+### 4. Optional: Windows Printer Support (pywin32)
+After installing requirements, run:
+```bash
+python -m pip install --upgrade pywin32
+python Scripts/pywin32_postinstall.py -install
+```
+
+This enables native Windows printer detection.
+
+## Running the App
+
+### From Command Prompt
+```bash
+venv\Scripts\activate
+python label_printer_gui.py
+```
+
+### Create Shortcut (Optional)
+Create a batch file `run_app.bat`:
+```batch
+@echo off
+call venv\Scripts\activate.bat
+python label_printer_gui.py
+pause
+```
+
+Then double-click the batch file to run the app.
+
+## Features
+
+✅ **Cross-Platform GUI** - Works on Windows, Linux, and macOS
+✅ **Barcode Generation** - Automatic Code128 barcode creation
+✅ **PDF Output** - High-quality PDF labels stored in `pdf_backup/` folder
+✅ **Printer Support** - Automatic printer detection (Windows, Linux, macOS)
+✅ **Input Validation** - 25-character limit with real-time validation
+✅ **PDF Backup** - All generated labels automatically saved
+
+## Printer Setup
+
+### Windows
+1. Go to Settings → Devices → Printers & Scanners
+2. Add your label printer
+3. Run the app - printer will be auto-detected
+4. Select printer from dropdown
+
+### Alternative (No Printer)
+- Select "PDF" option
+- Labels will be saved to `pdf_backup/` folder
+- Open and print from any PDF viewer
+
+## Troubleshooting
+
+### "No Printers Found"
+- This is normal - select "PDF" option
+- You can print PDFs manually from the backup folder
+- Or install your printer driver
+
+### Windows Defender Warning
+- Click "More info" → "Run anyway"
+- This is safe - the app is open-source
+
+### Missing Dependencies
+```bash
+pip install --upgrade pip
+pip install -r requirements_windows.txt
+```
+
+### Port Already in Use
+If you get an error about ports, restart your computer or:
+```bash
+python -m pip uninstall -y pywin32
+python -m pip install pywin32
+```
+
+## File Structure
+
+```
+Label-design/
+├── label_printer_gui.py      # Main GUI application
+├── print_label.py            # Print functionality
+├── print_label_pdf.py        # PDF generation
+├── requirements_windows.txt  # Windows dependencies
+├── pdf_backup/               # Stored PDF labels
+├── venv/                     # Virtual environment
+└── documentation/            # Documentation files
+```
+
+## Tips
+
+- **Character Limit**: Each field supports up to 25 characters (barcode limit)
+- **Quantity Field**: Only numbers allowed
+- **PDF Backup**: All labels automatically saved with timestamp
+- **Cross-Platform**: Same code runs on Windows, Linux, and macOS
+
+For more information, see the documentation folder.