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/Main.qml

@@ -0,0 +1,20 @@
+import QtQuick.Window
+import ISC
+
+// ===== App Window Shell =====
+Window {
+    // ===== Window Dimensions =====
+    width: 1400
+    height: 820
+    minimumWidth: 1100
+    minimumHeight: 700
+    visible: true
+    title: qsTr("ISenseCloud")
+
+    // Keep the window file very small.
+    // Styling and layout now live in HomePage.qml.
+    // ===== Root Page Mount =====
+    HomePage {
+        anchors.fill: parent
+    }
+}