Add Electron Builder distribution config for Folio

- Configure AppImage (primary) + Debian .deb (secondary) into releases/.
- White Folio app icon, Office category, io.atitlan.folio appId, folio
  executable; no file associations (none invented).
- Bundle tests/selftest-scenario.js so the packaged binary can be exercised
  in place via FOLIO_SELF_TEST (inert during normal launches).

electron-builder.yml

@@ -6,6 +6,9 @@ directories:
 files:
   - dist/**
   - package.json
+  # Bundled so the packaged binary can be exercised in-place (FOLIO_SELF_TEST).
+  # It is inert during normal launches.
+  - tests/selftest-scenario.js
 linux:
   icon: build/icons
   category: Office