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

build.gradle.kts

@@ -1,5 +1,7 @@
 plugins {
     id("com.android.application") version "8.7.3" apply false
     id("org.jetbrains.kotlin.android") version "2.1.10" apply false
-    id("com.google.gms.google-services") version "4.4.2" apply false
+    id("org.jetbrains.kotlin.plugin.serialization") version "2.1.10" apply false
+    // Firebase deferred to Sprint 2 (needs google-services.json)
+    // id("com.google.gms.google-services") version "4.4.2" apply false
 }