build: wire desktop-bundle pipeline (CI matrix + per-platform installers)

Stand up the seamless-download path for non-technical buyers:

* .github/workflows/build.yml — matrix CI (mac/win/linux) that builds
  PyInstaller bundles and packages them per platform on tag push,
  attaching the resulting installers to a GitHub Release.
* build/installer.iss — Inno Setup script for the Windows installer
  (per-user install, optional desktop shortcut, runs on finish).
* build/macos/build_dmg.sh — wraps DataTools.app into a .dmg with a
  drag-to-/Applications layout.
* build/appimage/{AppRun,datatools.desktop,build.sh} — AppImage recipe.
* src/__init__.py — single source of truth for __version__; the spec
  reads it (was hardcoded), CI passes it through to all packagers.

Buyer download path now lives in the top-level README. Per-build
README documents the Phase 2 step (signing/notarization) that needs
the owner's Apple Developer + Windows code-signing credentials —
those are intentionally not in CI yet because they require setup
outside this repo.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

build/appimage/datatools.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Type=Application
+Name=DataTools
+Comment=Local CSV / Excel cleaning suite
+Exec=DataTools
+Icon=datatools
+Categories=Office;Utility;
+Terminal=false