feat: add Stepper component

[Artur-]

Implements a new Stepper navigation component that displays progress through a sequence of logical steps. The component integrates with the existing @vaadin/stepper web component and provides full Java API support.

Key features:

• Container component for managing stepper steps
• Support for horizontal and vertical orientations
• Step states: inactive, active, completed, error
• Navigation via href or Router navigation targets
• Automatic current page detection
• Step validation and progress tracking methods
• Theme variants (small size)
• Full accessibility with ARIA labels
• TestBench elements for UI testing
• Comprehensive unit test coverage (48 tests passing)
• Factory methods for creating steps with hrefs

🤖 Generated with Claude Code

[vaadin-bot]

TC Format Checker Report - 15:13 - 25 - Aug

There are 12 files with format errors

• To see a complete report of formatting issues, download the differences file

• To fix the build, please run mvn spotless:apply in your branch and commit the changes.

• Optionally you might add the following line in your .git/hooks/pre-commit file:

  mvn spotless:apply
  

Here is the list of files with format issues in your PR:

flow-components-bom/pom.xml
vaadin-stepper-flow-parent/pom.xml
vaadin-stepper-flow-parent/vaadin-stepper-flow-integration-tests/pom.xml
vaadin-stepper-flow-parent/vaadin-stepper-flow/pom.xml
vaadin-stepper-flow-parent/vaadin-stepper-flow/src/main/java/com/vaadin/flow/component/stepper/Step.java
vaadin-stepper-flow-parent/vaadin-stepper-flow/src/main/java/com/vaadin/flow/component/stepper/Stepper.java
vaadin-stepper-flow-parent/vaadin-stepper-flow/src/main/java/com/vaadin/flow/component/stepper/StepperVariant.java
vaadin-stepper-flow-parent/vaadin-stepper-flow/src/test/java/com/vaadin/flow/component/stepper/StepTest.java
vaadin-stepper-flow-parent/vaadin-stepper-flow/src/test/java/com/vaadin/flow/component/stepper/StepperTest.java
vaadin-stepper-flow-parent/vaadin-stepper-testbench/pom.xml
vaadin-stepper-flow-parent/vaadin-stepper-testbench/src/main/java/com/vaadin/flow/component/stepper/testbench/StepElement.java
vaadin-stepper-flow-parent/vaadin-stepper-testbench/src/main/java/com/vaadin/flow/component/stepper/testbench/StepperElement.java

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