feat(mentor): email all team members button on project detail

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

src/app/(mentor)/mentor/projects/[id]/page.tsx

@@ -344,6 +344,25 @@ function ProjectDetailContent({ projectId }: { projectId: string }) {
           </CardDescription>
         </CardHeader>
         <CardContent className="space-y-4">
+            {(() => {
+              const emails = (project.teamMembers ?? [])
+                .map((m) => m.user.email)
+                .filter((e): e is string => !!e)
+              if (emails.length === 0) return null
+              const mailto = `mailto:${emails.join(',')}?subject=${encodeURIComponent(
+                `MOPC Mentorship — ${project.title}`,
+              )}`
+              return (
+                <div className="flex justify-end">
+                  <Button variant="outline" size="sm" asChild>
+                    <a href={mailto}>
+                      <Mail className="mr-2 h-4 w-4" />
+                      Email all team members
+                    </a>
+                  </Button>
+                </div>
+              )
+            })()}
           {/* Team Lead */}
           {teamLead && (
             <div className="p-4 rounded-lg border bg-muted/30">