fontFamily in Embedding SDK theme should not auto-quote the value

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**What problem will this feature request solve?**

When using defineMetabaseTheme with the fontFamily option, the getFontFamilyValue() function in `frontend/src/metabase/utils/fonts.ts` always wraps the entire value in double quotes before setting it as the CSS variable --mb-default-font-family. Per the MDN docs on font-family, quotes are only required for font names containing whitespace — the developer should determine when quotes are needed, not the framework. This breaks multiple fallback fonts (`"Lato, Roboto"` becomes a single font name instead of two), double-quotes pre-quoted names (`"Helvetica Neue"` becomes `""Helvetica Neue""`), and makes the hardcoded fallback chain unconfigurable

**Describe the solution you'd like.**
The fontFamily value from the theme config should be passed through to the CSS variable --mb-default-font-family without automatic quoting. This would let developers control their own font stacks with proper fallbacks, avoid double-quoting, and be more aligned with how CSS font-family actually works. Single font names like Lato would still work fine since quotes are optional in CSS for names without spaces.

**How does this feature request impact you?**
It blocks us from using proper font fallback chains in our embedded dashboards.

Start in frontend/src/metabase/utils/fonts.ts at getFontFamilyValue(), then trace where it sets --mb-default-font-family for defineMetabaseTheme. Check how the current value is transformed before reaching the CSS variable. Done means theme-provided font stacks pass through without automatic quoting, including fallback lists and already-quoted names.

Written by the indexing model from the issue text.