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

.gitignore

@@ -5,6 +5,9 @@ __pycache__/
 *.pyd
 *.so
 *.a
+*.egg-info/
+.pytest_cache/
+.ruff_cache/
 
 # Virtual environment
 .venv/
@@ -14,10 +17,11 @@ env/
 # IDE
 .vscode/
 .idea/
+.qtcreator/
 *.swp
 *.swo
 *~
-
+docs
 # OS
 .DS_Store
 Thumbs.db
@@ -34,4 +38,6 @@ build/
 dist/
 *.spec
 *.icns
-*.ico
+*.ico
+# Superpowers brainstorming visual companion
+.superpowers/