Add application icon support for GUI and compiled executables

Added platform-specific icon support for both the running application
and compiled executables:

New files:
- create_icons.py: Script to convert PNG to platform-specific formats
  - Generates .ico for Windows (16, 32, 48, 256px sizes)
  - Generates .iconset for macOS (ready for iconutil conversion)
- LocalTranscription.png: Source icon image
- LocalTranscription.ico: Windows icon file (multi-size)
- LocalTranscription.iconset/: macOS icon set (needs iconutil on macOS)

GUI changes:
- main.py: Set application-wide icon for taskbar/dock
- main_window_qt.py: Set window icon for GUI window

Build configuration:
- local-transcription.spec: Use platform-specific icons in PyInstaller
  - Windows builds use LocalTranscription.ico
  - macOS builds use LocalTranscription.icns (when generated)

To generate macOS .icns file on macOS:
  iconutil -c icns LocalTranscription.iconset

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

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

gui/main_window_qt.py

@@ -100,6 +100,12 @@ class MainWindow(QMainWindow):
         self.setWindowTitle("Local Transcription")
         self.resize(900, 700)
 
+        # Set application icon
+        icon_path = Path(__file__).parent.parent / "LocalTranscription.png"
+        if icon_path.exists():
+            from PySide6.QtGui import QIcon
+            self.setWindowIcon(QIcon(str(icon_path)))
+
         # Update splash
         self._update_splash("Creating user interface...")