fix(docker): run on Java 26 to match the build toolchain

The app is compiled with toolchain 26 (class file 70), but the runtime
image was eclipse-temurin:21 (max class file 65), which caused
UnsupportedClassVersionError at startup.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -23,7 +23,7 @@ RUN ./gradlew copyAngularBuild --no-daemon --info
 RUN ./gradlew bootJar -x test --no-daemon
 
 # 2. Финальный образ (Runtime stage)
-FROM eclipse-temurin:21-jdk-jammy
+FROM eclipse-temurin:26-jdk-jammy
 
 # Здесь оставляем все утилиты управления контейнерами
 RUN apt-get update && \