ChemSearch v1.13.6 - F-Droid-ready build support.

What's New

• Added a separate F-Droid build variant.
• GitHub builds keep the built-in APK updater.
• F-Droid builds remove the GitHub updater and installer permission.
• AI providers remain available as optional user-configured features.
• Added F-Droid metadata and store listing text.
• Moved app version data into version.properties for cleaner release handling.