feat: update nextjs-shadcn to Next 15.4

[sheepsteak]

ON HOLD: we need to implement AsyncLocalStorage in WebContainer before we can upgrade from the current Next.js version we use.

Created a brand new Next.js/Shadcn project and copied across necessary files from existing nextjs-shadcn.

Latest version we can do for now is Next.js 15.4 due to later versions using AsyncLocalStorage which is not implemented in WebContainer yet. See #122 (comment) for Next.16 errors.

[bolt-new-by-stackblitz]

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[sheepsteak]

With default next dev we get this output:

Error: Turbopack is not supported on this platform (linux/x64) because native bindings are not available. Only WebAssembly (WASM) bindings were loaded, and Turbopack requires native bindings.

To use Next.js on this platform, use Webpack instead:
  next dev --webpack

Trying next dev --webpack then results in:

## Error Type
Runtime InvariantError

## Error Message
Invariant: Expected workStore to be initialized. This is a bug in Next.js.

Next.js version: 16.2.6 (Webpack)