fix(registry): add Accept: application/json header to fetchRegistry

[cyphercodes]

When fetching from registry URLs, the CLI now sends an Accept: application/json header. This allows custom registries to use content negotiation to serve JSON to the CLI while serving HTML documentation to browsers at the same URL.

Problem

When using shadcn add with a custom registry URL, the CLI doesn't send an Accept header. This prevents registries from using standard HTTP content negotiation to serve different content types based on the client.

Solution

Add Accept: application/json header to the fetch request in fetchRegistry.

Testing

• Verified the header is included in requests
• Allows registries to differentiate between CLI and browser requests

Fixes #10164

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[EthanThatOneKid]

Hi @cyphercodes, thanks for opening this!

I was working on the same fix and just submitted PR #10179. Since yours was first, I wanted to note that I've included a suite of unit tests in my PR. If you'd like to pull those tests into your PR to make it more robust for the maintainers, feel free! I'll leave mine open.