chore(build): update JVM target to 25, enforce stable kotlinx-serialization-json, and add bundle size budget checks

- Updated Kotlin compiler and JVM toolchain to target JVM 25 in all subprojects.
- Enforced stable `kotlinx-serialization-json:1.7.3` version to prevent resolution issues.
- Introduced JS bundle size budget checks for frontend shells with gzip support and reporting.
- Refined and reorganized Gradle module includes in `settings.gradle.kts`.
- Removed legacy and redundant dependency configurations for improved clarity.

backend/infrastructure/gateway/src/main/kotlin/at/mocode/infrastructure/gateway/error/ProblemDetailsExceptionHandler.kt

@@ -2,7 +2,7 @@ package at.mocode.infrastructure.gateway.error
 
 import com.fasterxml.jackson.databind.ObjectMapper
 import org.slf4j.LoggerFactory
-import org.springframework.boot.web.reactive.error.ErrorWebExceptionHandler
+import org.springframework.boot.webflux.error.ErrorWebExceptionHandler
 import org.springframework.http.HttpStatus
 import org.springframework.http.MediaType
 import org.springframework.stereotype.Component

backend/infrastructure/gateway/src/main/kotlin/at/mocode/infrastructure/gateway/health/GatewayHealthIndicator.kt

@@ -42,6 +42,8 @@ class GatewayHealthIndicator(
     private val HEALTH_CHECK_TIMEOUT = Duration.ofSeconds(5)
   }
 
+  // KORREKTUR für Spring Boot 4: Die `health()`-Methode und ihr Rückgabetyp `Mono<Health>`
+  // erlauben keine Null-Werte mehr. Die Fragezeichen (?) wurden entfernt.
   override fun health(): Mono<Health> {
     val builder = Health.up()
     val details = mutableMapOf<String, Any>()

backend/infrastructure/gateway/src/main/kotlin/at/mocode/infrastructure/gateway/metrics/GatewayMetricsConfig.kt

@@ -4,7 +4,7 @@ import io.micrometer.core.instrument.Counter
 import io.micrometer.core.instrument.MeterRegistry
 import io.micrometer.core.instrument.Timer
 import io.micrometer.core.instrument.config.MeterFilter
-import org.springframework.boot.actuate.autoconfigure.metrics.MeterRegistryCustomizer
+import org.springframework.boot.micrometer.metrics.autoconfigure.MeterRegistryCustomizer
 import org.springframework.context.annotation.Bean
 import org.springframework.context.annotation.Configuration
 import org.springframework.web.server.ServerWebExchange