fix: use npmrc for legacy-peer-deps when available

[jacobparis]

If I'm using npm with React 19, the CLI asks me to use either --legacy-peer-deps or --force

> npx shadcn@latest add calendar                     
✔ Checking registry.
  Installing dependencies.

It looks like you are using React 19. 
Some packages may fail to install due to peer dependency issues in npm (see https://ui.shadcn.com/react-19).

How would you like to proceed?
  › Use --force
  › Use --legacy-peer-deps

These flags aren't required if I have a .npmrc file that sets them as a default, so this PR updates the CLI to check for that

> npx shadcn add calendar
✔ Checking registry.
⠹ Installing dependencies.
Using npm config: --legacy-peer-deps
✔ Installing dependencies.
? The file button.tsx already exists. Would you like to overwrite? › (y/N)

The relevant config is available in npm config list --json so this will match whatever npm is going to use.

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