epic: Minify release builds

[ovitrif]

Goal: release builds shrink code and resources with R8.

App

• Set isMinifyEnabled = true on the release build type.
• Set isShrinkResources = true on the release build type.
• Keep JNI native method names and their descriptor classes in app/proguard-rules.pro.
• Set the Kotlin compiler flag -Xannotation-default-target=param-property.

Libraries

Each consumed UniFFI Android AAR ships targeted R8 consumer keep rules so R8 full mode keeps JNA Structure fields, Callback methods, and the UniFFI JNA library symbols.

Acceptance

• A minified release build installs and runs code from the rust libs without runtime crashes.
• Each consumed UniFFI AAR contributes a non-empty proguard.txt to that build.