Feature: Add password change functionality for admin and customers

- Add customer password change page at /account/change-password
- Add admin password change page at /admin/change-password
- Create AdminUser database table for storing admin credentials
- Admin login now checks database first, falls back to env vars
- Update admin auth to support bcrypt password hashing
- Add change password links to customer account and admin nav
- Fix customer login logs back link (was pointing to non-existent /admin)
- Add success/error message handling for password changes

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

prisma/schema.prisma

@@ -449,6 +449,15 @@ model OrderItem {
   designImageDimensions String? // JSON array of {imageId, widthCm, heightCm} for each uploaded image
 }
 
+// Admin user account — stores admin credentials (can override env vars if present)
+model AdminUser {
+  id           String   @id @default(cuid())
+  username     String   @unique
+  passwordHash String
+  createdAt    DateTime @default(now())
+  updatedAt    DateTime @updatedAt
+}
+
 // Audit log for admin logins — tracks when the admin panel is accessed
 model AdminLoginLog {
   id        String   @id @default(cuid())