test(02-01): add failing serialization test for MeResponse DTO

RED phase of TDD task 02-01-01. Locks the wire-format contract for
GET /api/v1/me before the DTO exists:

- camelCase JSON keys (id, sub, email, displayName) per D-27
- ignoreUnknownKeys forward compat for Phase 3 householdId per D-28
- MeResponse.toUser() one-to-one mapping

Wires kotlinx.serialization into shared/build.gradle.kts (api scope so
both client and server inherit the @Serializable runtime) and adds the
kotlinx-serializationJson catalog alias. The shared module remains
pure: only kotlin stdlib + kotlinx.serialization-json are pulled into
commonMain (D-19 / INFRA-06 still holds).

Test currently fails: MeResponse and User unresolved; GREEN follows.

gradle/libs.versions.toml

@@ -28,6 +28,9 @@ spotless = "8.4.0"
 kotlin-test = { module = "org.jetbrains.kotlin:kotlin-test", version.ref = "kotlin" }
 kotlin-testJunit = { module = "org.jetbrains.kotlin:kotlin-test-junit", version.ref = "kotlin" }
 junit = { module = "junit:junit", version.ref = "junit" }
+
+# kotlinx.serialization (shared DTOs — D-27)
+kotlinx-serializationJson = { module = "org.jetbrains.kotlinx:kotlinx-serialization-json", version.ref = "kotlinx-serialization" }
 androidx-core-ktx = { module = "androidx.core:core-ktx", version.ref = "androidx-core" }
 androidx-testExt-junit = { module = "androidx.test.ext:junit", version.ref = "androidx-testExt" }
 androidx-espresso-core = { module = "androidx.test.espresso:espresso-core", version.ref = "androidx-espresso" }