feat(admin): replace window.prompt with proper modal for required review comments

[cdcore09]

Summary

apps/admin/src/pages/events/EventDetailPage.tsx and apps/admin/src/pages/announcements/AnnouncementDetailPage.tsx use window.prompt to collect the required comment for reject and request_changes transitions. Works but UX is rough — no validation, no formatting, no cancel-without-typing, blocks the JS thread.

Requirements

• Build a reusable <RequiredCommentModal> component (or use an existing modal primitive if one exists in apps/admin/src/components/)
• Replace the two window.prompt calls in EventDetailPage and AnnouncementDetailPage
• Validate non-empty before submit; show inline error if empty
• Add to any future artifact detail page (Plan 4 forms, Plan 5 broadcast UIs)

Context

Flagged in the Plan 3 review. Forms (Plan 4) will add another detail page with the same pattern — fix before that lands so the new code copies the modal instead of window.prompt.