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
jack