refactor: implement polymorphic Button and improve accessibility

[birdcoin0]

Description

Closes #55

This PR addresses the requested audit of the Button component and its call sites across the codebase to ensure semantic HTML, proper accessibility, and polymorphic rendering.

Changes

• Button Polymorphism: Refactored Button.tsx to be a polymorphic component using forwardRef. It now supports the as prop, allowing it to render as <a> or <Link> when navigation is required, rather than forcing a <button> tag.
• Accessibility Improvements:
  • Implemented aria-busy for loading states. This allows screen readers to distinguish between a "loading/in-flight" state and a "disabled/invalid" state.
  • Added forwardRef to ensure proper ref forwarding, critical for accessibility and integration with UI libraries.
• Call-site Audits:
  • Updated Navbar.tsx and ConnectPanel.tsx to use semantic link-style buttons for GitHub OAuth/redirects.
  • Updated IssueActions.tsx to use the new isLoading prop instead of disabled for async actions, improving the screen reader experience.

Verification

• Verified that all existing visual variants and sizes render correctly.
• Confirmed that navigation CTAs (like "Connect GitHub") now support middle-click-to-open and right-click-copy-link behavior.
• Manually audited loading/disabled states for screen reader compliance.

[chonilius]

fix ci