feat: enhance BountyCard component with hover effects, dark mode, accessibility

[tarai-dl]

Summary

Enhances the Bounty Card component (#1) with improved hover effects, dark mode support, accessibility, and a richer demo page.

Changes

• Hover effects: shadow-lg lift, border color shift to brand, subtle translate-y, title color change
• Dark mode: Full dark mode support for difficulty badges, progress bar, cards, and text
• Accessibility: aria-labels on card and progress bar, role=progressbar, keyboard navigation (Enter/Space)
• Progress bar: Gradient styling, clamped to 0-100%, smooth transition animation
• Demo page: Added difficulty variants section and progress states section showing all component variations
• Responsive: sm:grid-cols-2, lg:grid-cols-3 grid layouts across breakpoints

Props

type BountyCardProps = {
  title: string;
  reward: number;
  tags: string[];
  difficulty: Easy | Medium | Hard;
  progress: number;
  onClick?: () => void;
};

Demo

Desktop and mobile responsive layout verified. No TypeScript errors in bounty-card files (pre-existing error in create-bounty-form.tsx is unrelated).

Closes #1