Add PyInstaller support for standalone Windows executable

- Add build_exe.py script for easy executable generation
- Add PYINSTALLER_GUIDE.md with comprehensive instructions
- Updated label_printer.spec for PyInstaller configuration
- Tested build process - executable builds successfully
- Created dist/LabelPrinter with all dependencies bundled
- File size: ~200MB (includes Python runtime and all libraries)
- No Python installation required to run the exe

LabelPrinter.spec

@@ -0,0 +1,38 @@
+# -*- mode: python ; coding: utf-8 -*-
+
+
+a = Analysis(
+    ['label_printer_gui.py'],
+    pathex=[],
+    binaries=[],
+    datas=[],
+    hiddenimports=['kivy', 'kivy.core.window', 'kivy.core.text', 'kivy.core.image', 'kivy.uix.boxlayout', 'kivy.uix.gridlayout', 'kivy.uix.label', 'kivy.uix.textinput', 'kivy.uix.button', 'kivy.uix.spinner', 'kivy.uix.scrollview', 'kivy.uix.popup', 'kivy.clock', 'kivy.graphics', 'PIL', 'barcode', 'reportlab', 'print_label', 'print_label_pdf'],
+    hookspath=[],
+    hooksconfig={},
+    runtime_hooks=[],
+    excludes=[],
+    noarchive=False,
+    optimize=0,
+)
+pyz = PYZ(a.pure)
+
+exe = EXE(
+    pyz,
+    a.scripts,
+    a.binaries,
+    a.datas,
+    [],
+    name='LabelPrinter',
+    debug=False,
+    bootloader_ignore_signals=False,
+    strip=False,
+    upx=True,
+    upx_exclude=[],
+    runtime_tmpdir=None,
+    console=False,
+    disable_windowed_traceback=False,
+    argv_emulation=False,
+    target_arch=None,
+    codesign_identity=None,
+    entitlements_file=None,
+)