fix: suppress spurious font warnings during R3F teardown

[sawa-zen]

Problem

When React Three Fiber unmounts a component tree, removeChild calls parent.object.remove(child.object) on each three.js object. This triggers uikit's "removed" event handler (updateParentState), which calls properties.setEnabled(false), resetting all property signals to their defaults.

Since computedFont uses a raw effect() (not controlled by abortController), it re-evaluates when fontFamiliesSignal changes to undefined. The effect falls back to defaultFontFamiles ({ inter }), and if the component had a non-default fontFamily (e.g. "ja" for Japanese), it logs:

• unknown font family "ja". Available font families are "inter". Falling back to "inter".
• Missing glyph info for character "ラ" (for each character not in the inter font)

These warnings are harmless — the component is being destroyed — but noisy, especially with CJK text that produces one warning per character.

Fix

Skip font loading when fontFamiliesSignal.value is undefined. This value being undefined reliably indicates a fallback/teardown state (properties have been reset) rather than an explicit font configuration. In normal operation, fontFamiliesSignal.value is always set (either from own props or inherited from parent).

This prevents both the "unknown font family" error and the subsequent "Missing glyph info" warnings.

Reproduction

1. Create a uikit Container with fontFamilies including a non-Latin font
2. Add Text children with fontFamily set to that font
3. Unmount the component tree (e.g. navigate away in a React app)
4. Observe console warnings during unmount