Build fixes + unit tests for Sprint 1

- Fix settings.gradle.kts (dependencyResolutionManagement)
- Add gradle-wrapper.properties + gradle.properties
- Defer Firebase/FCM to Sprint 2 (no google-services.json yet)
- Add placeholder ic_clippy drawable
- Add test deps (JUnit, coroutines-test)
- VoiceStateMachineTest: 14 tests covering happy path, interruption, cancellation, invalid transitions, reset
- ResponseDedupTest: basic dedup cache tests
- Add kotlinx-serialization plugin for Ktor

gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,7 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.9-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists