Restructure into src/ layout under pygui package

Move all application source under src/pygui/ and rewire imports,
build config, and QML module path to match.
- Relocate backend/, serialcomm/, and the ISC QML module into
  src/pygui/; convert main.py into pygui/__main__.py with a main()
  entry point (run via `python -m pygui` or the new `isc` script)
- Rewrite absolute imports: backend.* -> pygui.backend.*,
  serialcomm.* -> pygui.serialcomm.* (source + tests)
- Move app icons (isc.ico/icns) into packaging/
- Update README and ISC.qmlproject to the new paths

src/pygui/__main__.py

@@ -0,0 +1,52 @@
+import sys
+from pathlib import Path
+
+from PySide6.QtQml import QQmlApplicationEngine
+from PySide6.QtQuickControls2 import QQuickStyle
+from PySide6.QtWidgets import QApplication
+
+from pygui.backend.device_controller import DeviceController
+from pygui.backend.local_settings import LocalSettings
+from pygui.backend.local_settings_model import LocalSettingsModel
+from pygui.backend.file_browser import FileBrowser
+
+
+# ===== Application Entry Point =====
+def main() -> int:
+    # ===== UI Style Setup =====
+    # Use a non-native controls style so our custom QML button backgrounds are supported.
+    QQuickStyle.setStyle("Basic")
+
+    # ===== Qt Bootstrap =====
+    # Minimal Qt bootstrap: load the ISC QML module and exit if it fails.
+    app = QApplication(sys.argv)
+    engine = QQmlApplicationEngine()
+
+    # ===== Shared Models =====
+    settings_model = LocalSettingsModel()
+    select_file_dialog_model = FileBrowser()
+    settings_model.loadSettings()
+    engine.rootContext().setContextProperty("settingsModel", settings_model)
+    engine.rootContext().setContextProperty("file_browser", select_file_dialog_model)
+
+    # ===== Device Controller (serial comm) =====
+    data_dir = str(settings_model._data_dir)
+    raw_settings = LocalSettings.read_settings(data_dir)
+    device_controller = DeviceController(raw_settings, data_dir)
+    engine.rootContext().setContextProperty("deviceController", device_controller)
+
+    # ===== QML Startup =====
+    # The "ISC" QML module lives alongside this file (src/pygui/ISC), so the
+    # package directory is the import path the engine searches for qmldir.
+    engine.addImportPath(Path(__file__).parent)
+    engine.loadFromModule("ISC", "Main")
+
+    # ===== Exit Handling =====
+    if not engine.rootObjects():
+        return -1
+
+    return app.exec()
+
+
+if __name__ == "__main__":
+    sys.exit(main())