chore(deps): update all libraries and Gradle to latest stable versions

Bump the version catalog, Gradle wrapper, and convention plugins to the
latest stable releases. Verified with `./gradlew assembleDebug test`
(BUILD SUCCESSFUL, 21 unit tests pass).

Toolchain:
- Gradle 9.1.0 -> 9.5.1
- AGP 9.0.1 -> 9.2.1
- Kotlin 2.3.20 -> 2.4.0

Libraries:
- androidx-core 1.18.0 -> 1.19.0, appcompat 1.7.0 -> 1.7.1,
  fragment 1.8.5 -> 1.8.9, navigation 2.9.0 -> 2.9.8
- compose-bom 2026.03.01 -> 2026.05.01, material 1.12.0 -> 1.14.0
- coroutines 1.10.2 -> 1.11.0, serialization 1.8.1 -> 1.11.0,
  collections-immutable 0.3.8 -> 0.5.0
- koin 4.1.0 -> 4.2.1, ktor 3.1.3 -> 3.5.0, coil 3.1.0 -> 3.5.0
- JUnit 5.11.4 -> 6.1.0, turbine 1.2.0 -> 1.2.1, mockk 1.14.3 -> 1.14.11

Required side-effect:
- compileSdk 36 -> 37 (mandated by androidx.core 1.19.0); targetSdk
  left at 36.

Also refresh stale JUnit 5 / AGP 9.0 / compileSdk 36 references in the
README and convention-plugin docs.

feature/characters/domain/src/test/kotlin/com/example/architecture/feature/characters/domain/usecase/GetCharactersPageUseCaseTest.kt

@@ -18,7 +18,7 @@ import org.junit.jupiter.api.Test
 /**
  * Tests for the (thin pass-through) [GetCharactersPageUseCase]: it must forward the requested page to
  * the repository and return its result verbatim - success and error alike. Pure JVM test on the
- * JUnit 5 platform (see DomainModuleConventionPlugin); the [CharacterRepository] collaborator is a
+ * JUnit Platform (see DomainModuleConventionPlugin); the [CharacterRepository] collaborator is a
  * MockK mock, stubbed with `coEvery` and verified with `coVerify`.
  */
 class GetCharactersPageUseCaseTest {