chore: resolve merge conflicts, keep Spring Boot 4.0.6 version
Kept our version over remote (security hardening PR #12) for all conflicts: - pom.xml: Spring Boot 4.0.6 vs 3.4.1, new test deps - SecurityConfig: new csrf/package API for SB4 - GlobalExceptionHandler: full handler set with SLF4J logging - AppUserController/ExpenseListController: clean delegation to GlobalExceptionHandler - Test files: SB4 package paths, extra test coverage Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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