Add PyInstaller spec and Gitea Actions CI for Linux/Windows binaries

- run.py: top-level entry point for PyInstaller (relative imports need it)
- psp-preserve.spec: onedir build bundling DATs, loader, cover bundles, icons
- requirements-build.txt: pinned build deps (PySide6 6.8.3 for Windows ICU fix)
- .gitea/workflows/build.yml: builds linux + windows artifacts on tags/manual
- gitignore build/ dist/

run.py

@@ -0,0 +1,10 @@
+"""PyInstaller entry point.
+
+Runs the GUI the same way ``python -m app.main`` does, but as a plain script
+so PyInstaller can build it (relative imports inside ``app`` won't resolve when
+a module file is executed directly, hence this top-level launcher).
+"""
+from app.main import main
+
+if __name__ == "__main__":
+    raise SystemExit(main())