feat(frontend): replace Next.js with TanStack Start

[Liinkiing]

What changed

• Replaced the Next.js frontend with a TanStack Start (SPA mode) app.
• Restored Storybook using the Vite-based Storybook framework.
• Kept existing shared UI/theme/Apollo/GraphQL codegen + Panda CSS, updated routing/navigation to TanStack Router.

Why

• Evaluate TanStack Start as the main frontend framework for this repo.

How to test

• App:
  • pnpm --filter=frontend dev
  • Open http://localhost:3000
• Storybook:
  • pnpm --filter=frontend storybook
  • Open http://localhost:6006

Notes

• pnpm --filter=frontend lint, pnpm --filter=frontend ts:check, pnpm --filter=frontend build pass locally.
• pnpm --filter=frontend build-storybook passes locally.