fix: make Button polymorphic with distinct loading/disabled a11y states (#55)

[rafaio1]

Fixes #55.

• Rewrite Button as a polymorphic component supporting as prop (e.g. as="a" or as={Link}) so navigation CTAs render correct anchor semantics instead of imperative onClick redirects.
• Add distinct loading state using aria-busy="true" and a visual spinner overlay that keeps the element in the tab order, separate from native disabled which removes it from the accessibility tree entirely.
• Forward ref correctly for all variants and element types.
• Preserve all existing visual variants (primary, secondary, ghost, outline) and sizes (sm, md, lg).
• TypeScript compiles cleanly with npx tsc --noEmit.

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