feat(registry): add @better-auth-ui to registry directory

[AlexProgrammerDE]

This pull request adds @better-auth-ui to the registry directory with its homepage, style-aware URL template, description, and official SVG mark.

Registry: https://better-auth-ui.com/r/registry.json

Summary

• Adds @better-auth-ui to the registry directory.
• Uses https://better-auth-ui.com/r/{style}/{name}.json so shadcn resolves the matching Radix UI or Base UI registry variant.
• Includes the Better Auth UI SVG mark with theme-aware coloring.

Validation

• Confirmed the registry catalog and representative Radix UI and Base UI item endpoints return valid JSON.
• Verified the directory JSON, entry uniqueness, local ordering, and diff formatting.

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[AlexProgrammerDE]

This is ready for merge.