chore: Set up monorepo structure using pnpm workspaces

[junkisai]

Summary

I've implemented a monorepo structure using pnpm workspace.
Currently, we only have the service-site app, but we anticipate adding more applications in the future. The monorepo approach was chosen because we expect to use similar UI components across multiple applications.

.
└── frontend/
    ├── package.json
    ├── apps/
    │   │── service-site/
    │   │   ├── package.json
    │   │   └── app/
    │   │       ├── page.tsx
    │   │       └── blog/
    │   │           └── [id]/
    │   │               └── page.tsx
    │   └── build-app
    └── packages/
        └── ui

Related Issue

Changes

Testing

Other Information

[MH4GF]

LGTM 🚀

[MH4GF]

👍🏻