feat(admin): send mentorship welcome/reminder button on mentoring rounds

Adds a sky-accented "Send Welcome / Reminder" button to the Notifications
grid in the round page, visible only on MENTORING rounds. Wires into
trpc.mentor.previewMentorshipWelcome / sendMentorshipWelcome via the
shared EmailPreviewDialog with optional custom note support.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/(admin)/admin/rounds/[roundId]/page.tsx

@@ -125,6 +125,7 @@ import { ConfigSectionHeader } from '@/components/admin/rounds/config/config-sec
 import { NotifyAdvancedButton } from '@/components/admin/round/notify-advanced-button'
 import { NotifyRejectedButton } from '@/components/admin/round/notify-rejected-button'
 import { BulkInviteButton } from '@/components/admin/round/bulk-invite-button'
+import { SendMentorshipWelcomeButton } from '@/components/admin/round/send-mentorship-welcome-button'
 import { AdvancementSummaryCard } from '@/components/admin/round/advancement-summary-card'
 import { FinalizationTab } from '@/components/admin/round/finalization-tab'
 
@@ -1442,6 +1443,7 @@ export default function RoundDetailPage() {
                       <NotifyAdvancedButton roundId={roundId} />
                       <NotifyRejectedButton roundId={roundId} />
                       <BulkInviteButton roundId={roundId} />
+                      {isMentoring && <SendMentorshipWelcomeButton roundId={roundId} />}
                     </div>
                   </div>
                 )}