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/ISC/qmldir

@@ -0,0 +1,9 @@
+# ===== ISC QML Module =====
+module ISC
+
+# ===== Root Components =====
+Main 1.0 Main.qml
+HomePage 1.0 HomePage.qml
+
+# ===== Singleton Theme =====
+singleton Theme 1.0 Theme.qml