fix(cli): skip unavailable components for add --all

[meetdhruva]

Summary

• preflight add --all candidates against the configured style
• skip candidates that return RegistryNotFoundError and warn with their names
• preserve failures for explicit add <component> calls and all non-404 registry errors
• add coverage for a global-index item that is missing from the selected style

Closes #11562

Validation

• pnpm --filter shadcn exec vitest run src/commands/add.test.ts
• pnpm --filter shadcn typecheck

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