Feature: Add unread message notifications to admin badge

- Add isRead boolean field to Message model (default: false)
- Create migration to update database schema
- Update admin notifications to count unread messages
- Display unread message count in notification badge tooltip
- Admins now see all notifications: pending orders, design approvals, photo requests, unread messages
- Color-coded tooltip: orange (orders), blue (designs), clay (photos), pink (messages)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

prisma/schema.prisma

@@ -175,6 +175,7 @@ model Message {
   proof     DesignProof @relation(fields: [proofId], references: [id], onDelete: Cascade)
   sender    String // "ADMIN" | "CUSTOMER"
   body      String
+  isRead    Boolean     @default(false)
   createdAt DateTime    @default(now())
 }