Add application icon for all platforms

- Created icon generation script (create_icon.py)
- Generated PNG icon (512x512) with word search grid design
- Generated ICO file for Windows (multi-resolution)
- Updated PyInstaller spec to use platform-appropriate icons
- Updated .gitignore to exclude generated icon size variants

Icon features:
- Light blue gradient background
- 5x5 grid with letters spelling WORD, SEARCH, etc.
- Highlighted "WORD" in green to show found word
- Professional, clean design

Windows: icon.ico
macOS: icon.icns (generate on macOS with provided commands)
Linux: icon.png

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

word_search.spec

@@ -1,7 +1,18 @@
 # -*- mode: python ; coding: utf-8 -*-
 
+import sys
+import os
+
 block_cipher = None
 
+# Determine icon path based on platform
+if sys.platform == 'win32':
+    icon_file = 'icon.ico'
+elif sys.platform == 'darwin':
+    icon_file = 'icon.icns'  # Will need to be created on macOS
+else:
+    icon_file = 'icon.png'
+
 a = Analysis(
     ['word_search_generator.py'],
     pathex=[],
@@ -40,12 +51,13 @@ exe = EXE(
     target_arch=None,
     codesign_identity=None,
     entitlements_file=None,
+    icon=icon_file if os.path.exists(icon_file) else None,
 )
 
 # macOS app bundle
 app = BUNDLE(
     exe,
     name='WordSearch.app',
-    icon=None,
+    icon=icon_file if sys.platform == 'darwin' and os.path.exists(icon_file) else None,
     bundle_identifier='com.firebugit.wordsearch',
 )