fix(cli): allow apply in monorepo UI workspaces without framework config

[yogeshwaran-c]

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

Closes #10411

Running shadcn apply -c packages/ui (or bunx shadcn@latest apply --preset … -c packages/ui) from a shadcn-generated monorepo fails with:

We could not detect a supported framework at /.../packages/ui.
Visit https://ui.shadcn.com/docs/installation/manual to manually configure your project.
Once configured, you can use the cli to add components.

The target packages/ui has a valid components.json (it was scaffolded by shadcn init itself), but it has no framework config file (next.config.*, vite.config.*, etc.) because the UI library workspace intentionally doesn't ship one — those live in apps/web. preFlightInit then flags the workspace with UNSUPPORTED_FRAMEWORK and exits.

What is the new behavior?

preFlightInit now treats a provided options.existingConfig as proof the project has already been configured for shadcn, and skips the framework detection error in that case. The tailwind-CSS and import-alias checks continue to run normally, so misconfigured projects are still caught.

Practical effects:

• shadcn apply --preset … -c packages/ui now proceeds past preflight and applies the preset.
• Any re-init on a project that already has a components.json no longer fails with UNSUPPORTED_FRAMEWORK if the framework can't be auto-detected.
• First-time init behaviour is unchanged — without existingConfig the original monorepo detection + error message still apply.

Additional context

• A regression test was added at packages/shadcn/src/preflights/preflight-init.test.ts covering both the new branch (existingConfig present, no framework -> no error) and the pre-existing branch (no existingConfig, no framework -> exits with error).
• A patch changeset was added for shadcn.

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