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>
                 )}

src/components/admin/round/send-mentorship-welcome-button.tsx

@@ -0,0 +1,61 @@
+'use client'
+
+import { useState } from 'react'
+import { trpc } from '@/lib/trpc/client'
+import { toast } from 'sonner'
+import { Mail } from 'lucide-react'
+import { EmailPreviewDialog } from './email-preview-dialog'
+
+interface SendMentorshipWelcomeButtonProps {
+  roundId: string
+}
+
+export function SendMentorshipWelcomeButton({ roundId }: SendMentorshipWelcomeButtonProps) {
+  const [open, setOpen] = useState(false)
+  const [customMessage, setCustomMessage] = useState<string | undefined>()
+
+  const preview = trpc.mentor.previewMentorshipWelcome.useQuery(
+    { roundId, customNote: customMessage },
+    { enabled: open },
+  )
+
+  const sendMutation = trpc.mentor.sendMentorshipWelcome.useMutation({
+    onSuccess: (data) => {
+      toast.success(
+        `Sent ${data.sent} email${data.sent !== 1 ? 's' : ''}${data.failed ? ` (${data.failed} failed)` : ''}`,
+      )
+      setOpen(false)
+    },
+    onError: (err) => toast.error(err.message),
+  })
+
+  return (
+    <>
+      <button
+        onClick={() => setOpen(true)}
+        className="flex items-start gap-3 p-4 rounded-lg border border-l-4 border-l-sky-500 hover:-translate-y-0.5 hover:shadow-md transition-all text-left w-full"
+      >
+        <Mail className="h-5 w-5 text-sky-600 mt-0.5 shrink-0" />
+        <div>
+          <p className="text-sm font-medium">Send Welcome / Reminder</p>
+          <p className="text-xs text-muted-foreground mt-0.5">
+            Email all mentors &amp; teams how to use mentorship
+          </p>
+        </div>
+      </button>
+
+      <EmailPreviewDialog
+        open={open}
+        onOpenChange={setOpen}
+        title="Send Mentorship Welcome / Reminder"
+        description="Emails every mentor and team member in this round with their assignment and how to use the mentorship features. You can add an optional note below."
+        recipientCount={preview.data?.recipientCount ?? 0}
+        previewHtml={preview.data?.html}
+        isPreviewLoading={preview.isLoading}
+        onSend={(msg) => sendMutation.mutate({ roundId, customNote: msg })}
+        isSending={sendMutation.isPending}
+        onRefreshPreview={(msg) => setCustomMessage(msg)}
+      />
+    </>
+  )
+}