Fix default Tailwind v4 registry fallback

[raashish1601]

Summary

• Treat legacy default style with an empty Tailwind config the same as new-york, resolving it to the Tailwind v4 fallback style.
• Add coverage so default falls back only for Tailwind v4 configless projects, while legacy config-file projects and custom styles are preserved.

This addresses the styles/default/combobox.json 404 path reported in #10065 without changing legacy default projects that still have a Tailwind config file.

Test plan

• corepack pnpm exec vitest run packages/shadcn/src/registry/config.test.ts
• corepack pnpm --filter shadcn typecheck
• corepack pnpm exec prettier --check packages/shadcn/src/registry/config.ts packages/shadcn/src/registry/config.test.ts
• corepack pnpm --filter shadcn build
• git diff --check

Notes:

• corepack pnpm exec eslint packages/shadcn/src/registry/config.ts packages/shadcn/src/registry/config.test.ts is currently blocked locally because the repo has legacy .eslintrc.json config while the installed ESLint 9 CLI expects eslint.config.*.
• corepack pnpm --filter shadcn test has existing Windows-specific path snapshot/local-file failures outside this change; the targeted registry config test passed.

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