feat: dark IDE visual redesign

- switch to dark theme tokens: deep blue-black bg, indigo-cyan accent
- gradient hero headline, refined challenge list with hover states
- IDE-style file tabs, diff left-border colors, line hover highlight
- glass inline finding cards, gradient segmented control and buttons
- conic-gradient score ring, unified 160ms transitions and focus rings
- move source attribution into title badge row as inline pill

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: study8677