Add rainbow login and Niteshift setup

[kc-beard]

Summary

• replace the plain login surface with a full-spectrum rainbow background
• present the login form in a responsive frosted-glass card
• add matching rainbow treatments to the Umami mark, title, and login button
• install locked dependencies and generate the Prisma client during Niteshift setup
• declare the Next.js development server as the default supervised preview service

Verification

View runtime verification report

• visually checked the running login page at desktop size
• confirmed the layout fits a 390 × 844 viewport without horizontal overflow
• started the repository-declared web service and confirmed it is healthy with preview enabled
• requested /login from the declared service and received HTTP 200
• formatter and TypeScript checks pass

Screenshots

🌒 Run on Niteshift: View Task