✨ Add theme.json partials support

[retlehs]

Summary

• Automatically discovers and deep merges *.theme.js files into the generated theme.json
• Scans resources/ by default, configurable via partials option
• Supports two export formats: shorthand (blocks/elements) and full (styles: { ... })
• Registers partial files as config dependencies for dev server rebuilds

🤖 Generated with Claude Code

[retlehs]

For folks using Radicle, this feature replaces https://github.com/roots/radicle/blob/f0d33a303ab7abd437b06dd810e62d1c3c840af3/vite.config.js#L32-L79