Memoize ClerkProvider appearance prop to prevent unnecessary re-renders

[Copilot]

The appearance prop in ClerkProvider was creating a new object on every render, triggering unnecessary re-renders of ClerkClerkProvider and its children.

Changes

• Memoized the appearance object using React.useMemo with themeName as the dependency
• The appearance object is now only recreated when themeName changes

const appearance = React.useMemo(() => {
  const theme = themeName ? themes[themeName as keyof typeof themes] : undefined;
  return theme ? { baseTheme: theme } : undefined;
}, [themeName]);

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