Accessibility: FieldSet and Legend component

[stsrki]

Closes #6441

This PR adds first-class support for semantic form grouping in Blazorise with new independent FieldSet and Legend components. They render native fieldset and legend elements, have dedicated class provider methods across all supported providers, and integrate with existing accessibility options so grouped controls such as RadioGroup can automatically use the parent Legend as their accessible label through aria-labelledby. The PR also adds demo and documentation examples showing the recommended pattern for radio groups and related checkbox sets, while keeping Field and FieldLabel focused on single-input scenarios.

[njannink]

[x] Code Reviewed

General Test Remarks

• Was not able to test validation logic. Maybe add to ValidationsPage

Provider Specific Tests

• AntDesign
  • Alignment for vertical fields off
  
• Bootstrap
• Bootstrap Server
• Bootstrap5
• Bulma
  Legend missing and alignments off
  
• FluentUI2
• Material
  • padding / margin of elements is missing
  
• Tailwind

[njannink]

see comments in conversation

[stsrki]

The latest refactoring moves semantic grouping support onto the existing Field and Fields APIs, instead of treating FieldSet and Legend as specialized versions of Field and FieldLabel. Field and Fields now opt into grouped semantics explicitly, and their label components switch to rendering a legend when used in group mode. At the same time, FieldSet and Legend were simplified into standalone native wrappers that render only the fieldset and legend, without inheriting the behavior, accessibility wiring, or layout assumptions of Field and FieldLabel.

The reason for this split is to avoid forcing native fieldset and legend usage into the styling and behavioral model of a single-field container. A real fieldset can group many different kinds of controls, not just the scenarios that map cleanly to Field, so coupling those components too tightly would artificially limit their use. By reusing Field and Fields for the higher-level grouped API, and keeping FieldSet and Legend as dumb native elements with their own class-provider hooks for horizontal, validation, required-indicator, and screenreader styling, the API becomes clearer: the existing components handle Blazorise form composition, while the standalone wrappers remain available for direct semantic HTML when needed.

[stsrki]

@njannink, please recheck the API and UI

[stsrki]

PS. I'm thinking of renaming the Group parameter to SemanticGroup.

[njannink]

SemanticGroup says the same to me as Group. Maybe use (Is)FieldSet to really point at the underlying html that will be rendered?

[njannink]

• API Reviewed
• Documentation tested and reviewed

General Test Remarks

⚠️ Radio groups don't seem to generate the html semantic fieldset tag

Provider Specific Tests

• AntDesign
• Bootstrap
• Bootstrap Server
• Bootstrap5
• Bulma
• FluentUI2
  • legend tag not generated for radio group
• Material
• Tailwind