Config-aware theme resolution for plan site

Update the plan site's resolveThemeCss() to read refrakt.config.json when no --theme flag is provided, so the plan site automatically uses the project's installed theme.

When a theme package is specified in config, resolve its CSS (design tokens + base styles) and layer it under the plan rune CSS. When resolution fails or no config exists, fall back to the built-in default theme.

Acceptance Criteria

When --theme is omitted and refrakt.config.json exists with a theme field, that theme is used
Theme package is resolved via its npm package exports (package/html or package/base.css)
--theme flag still overrides config when provided
--theme default and --theme minimal still use built-in themes regardless of config
When config exists but theme resolution fails, falls back to built-in default with a warning
When no config exists, uses built-in default (standalone mode unchanged)
--theme lumina works as shorthand for @refrakt-md/lumina

Approach

In resolveThemeCss(), check if --theme was explicitly provided
If not, look for refrakt.config.json in the working directory (or --config path)
If config has theme, attempt to resolve it as an npm package
Load the theme's ThemeConfig from package/transform export and merge with plan rune configs
Load CSS from the theme package's base export
Fall back gracefully on any error