fix: admin role change, logo access, magic link validation, login help

- Add updateTeamMemberRole mutation for admins to change team member roles
- Allow any team member (not just lead) to change project logo
- Add visible "Add logo"/"Change" label under logo for discoverability
- Pre-check email existence before sending magic link (show error)
- Add "forgot which email" contact link on login page

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/api/auth/check-email/route.ts

@@ -0,0 +1,26 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { prisma } from '@/lib/prisma'
+
+/**
+ * Pre-check whether an email exists before sending a magic link.
+ * This is a closed platform (no self-registration) so revealing
+ * email existence is acceptable and helps users who mistype.
+ */
+export async function POST(req: NextRequest) {
+  try {
+    const { email } = await req.json()
+    if (!email || typeof email !== 'string') {
+      return NextResponse.json({ exists: false }, { status: 400 })
+    }
+
+    const user = await prisma.user.findUnique({
+      where: { email: email.toLowerCase().trim() },
+      select: { status: true },
+    })
+
+    const exists = !!user && user.status !== 'SUSPENDED'
+    return NextResponse.json({ exists })
+  } catch {
+    return NextResponse.json({ exists: false }, { status: 500 })
+  }
+}