Remove Karma testing framework

[ChristianHuehn]

Feature request

Description

As a developer, I want to replace Karma with a modern testing framework so that tests run faster and the tooling is better maintained.

Acceptance criteria

• Karma is completely removed from the project
• All existing tests are migrated to the new framework
• Test execution time is improved or at least maintained
• CI/CD pipeline is updated to use the new test runner
• Documentation is updated with new testing instructions

Assumptions & Exclusions

• Assumes we'll migrate to Jest or Vitest as replacement
• Existing test coverage should be maintained

Development notes (optional Task Breakdown)

• Choose replacement testing framework (Jest/Vitest)
• Migrate existing test configuration
• Update all test files to new framework syntax
• Update package.json scripts
• Update CI/CD configuration
• Remove Karma dependencies
• Update testing documentation

Open questions

• Which testing framework should replace Karma (Jest or Vitest)?
• Are there any tests that have framework-specific features?