Add note about label.

[polina-c]

Instruct to add label 'component: genui' to issues.

[gemini-code-assist]

Code Review

This pull request updates the CONTRIBUTING.md file to instruct contributors to apply the component: genui label when filing issues. The reviewer suggested a minor grammatical improvement to make the phrasing more idiomatic.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[github-actions]

Package publishing

If you have publishing permissions, you can use the links below to publish the changes after merging this PR.

Package	Version	Status	Publish tag (post-merge)
package:a2ui_core	0.0.1-wip002	ready to publish	a2ui_core-v0.0.1-wip002
package:genai_primitives	0.2.4	ready to publish	genai_primitives-v0.2.4
package:genui	0.9.2	(error) pubspec version (0.9.2) and changelog (0.10.0) don't agree
package:genui_a2a	0.9.0	(error) pubspec version (0.9.0) and changelog (0.10.0) don't agree
package:json_schema_builder	0.1.5	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.