feat(workspace): mentor + applicant message previews (§F.2)

mentor.getRecentMessages: last N unread messages from teams across all
of a mentor's assignments. Drives a Recent Messages card on /mentor.

applicant.getMentorConversationPreview: last 3 messages + unread count
for a given project. Drives a 'Conversation with [Mentor]' card on
/applicant — auto-hides when no mentor is assigned.

Both procedures use the existing MentorMessage(projectId, createdAt)
composite index — no new index needed.

Plan: docs/superpowers/plans/2026-04-28-pr6-multi-role-and-workspace-previews.md

src/app/(applicant)/applicant/page.tsx

@@ -17,6 +17,7 @@ import { Skeleton } from '@/components/ui/skeleton'
 import { Textarea } from '@/components/ui/textarea'
 import { CompetitionTimelineSidebar } from '@/components/applicant/competition-timeline'
 import { MentoringRequestCard } from '@/components/applicant/mentoring-request-card'
+import { MentorConversationCard } from '@/components/applicant/mentor-conversation-card'
 import { AnimatedCard } from '@/components/shared/animated-container'
 import { ProjectLogoUpload } from '@/components/shared/project-logo-upload'
 import { Progress } from '@/components/ui/progress'
@@ -401,6 +402,10 @@ export default function ApplicantDashboardPage() {
             </AnimatedCard>
           ))}
 
+          {/* Conversation with assigned mentor (auto-hides when no mentor assigned) */}
+          <MentorConversationCard projectId={project.id} />
+
+
           {/* Jury Feedback Card */}
           {totalEvaluations > 0 && (
             <AnimatedCard index={4}>