What this PR does / why we need it:

Adds control plane UI capabilities to the Feast web UI, allowing users to
create, edit, and delete entities, data sources, and feature views directly
from the browser.

Changes include:

• Form modals for creating/editing entities, data sources, and feature views
  (EntityFormModal, DataSourceFormModal, FeatureViewFormModal)
• Reusable form primitives: FormModal, TagsEditor, ValueTypeSelect,
  NameDescriptionOwnerFields, FeatureFieldEditor
• REST query hooks: useLoadEntitiesREST, useLoadDataSourcesREST,
  useLoadFeatureViewsREST
• Mutation hooks: useEntityMutations, useDataSourceMutations,
  useFeatureViewMutations
• UIVersionContext + UIVersionToggle component to toggle between read-only
  and control-plane UI modes
• Integrated apply/delete actions into existing overview and listing pages

Which issue(s) this PR fixes:

• Fixes #6192

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change