Add Fastlane build and test workflows

[andre487]

Summary

• make Fastlane the supported entry point for native and Android tests
• build debug and unsigned release APKs for pull requests and publish them as workflow artifacts
• keep release signing isolated to the tag-triggered release workflow
• add English and Russian Fastlane setup and command documentation
• remove the obsolete artwork launcher icon and document the canonical branding asset

Verification

• Fastlane 2.238.0 loads all configured lanes on Ruby 3.4
• Android unit tests, lint, debug assembly, and unsigned release assembly pass
• the CI release APK is verified as unsigned with apksigner
• Fastfile, workflow YAML, VS Code JSON, and repository diff validation pass

[andre487]

APK artifacts

• Download debug APK — signed with the disposable Android debug key
• Download unsigned release APK — verified unsigned

Artifacts expire after 14 days. Neither APK uses the MegaProxy release key.