Bring `LanguageSwitcherPill` to shadcn-registry quality. Depends on #58. - Spring-animated indicator (motion `layoutId`) replacing manual measurement + ResizeObserver; RTL handled by layout. - shadcn tokens; `variant` (default / outline), `size`, `showFlags`, `disabled`, `classNames` slots. - Backward compatible; radiogroup semantics and roving focus unchanged.
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