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

ISC.qmlproject

@@ -0,0 +1,16 @@
+import type QmlProjectModel 1.0
+
+QmlProject {
+    name: "ISC"
+    version: "1.0"
+    mainFile: "src/pygui/ISC/Main.qml"
+
+    dependencies {
+        module: "QtQuick"
+        minimumVersion: "6.0.0"
+    }
+
+    importPaths: [
+        "src/pygui"
+    ]
+}