Polish small UX items for group AI budget management #26401

Linear: https://linear.app/codercom/issue/AIGOV-444/polish-small-ux-items-for-group-ai-budget-management-before-code

UX checklist:

• Revisit the empty/edge-case design for the AI budget section in site/src/pages/GroupsPage/GroupSettingsPageView.stories.tsx.
• Confirm the final UX for this state with Design/Product.
• Update Storybook coverage for the finalized UX states.
• Polish copy/helper text for capped, uncapped, and disabled (0) budget states.
• Review validation, loading, and error states for low-risk UX improvements before code freeze.

Context:

• PR: feat: add group AI budget management UI #26375
• Discussion: feat: add group AI budget management UI #26375 (comment)

Generated by Coder Agents on behalf of @EhabY.