feat: add copy-to-clipboard button on result card URL

[GeethaBurigalla]

Closes #179

Changes

• Added a "Copy" button next to the URL shown in the result card
• Uses navigator.clipboard.writeText() with a document.execCommand('copy') fallback for unsupported browsers
• Button shows a checkmark + "Copied" label for 2 seconds after a successful copy, then reverts
• Styled to match the existing cyan accent / result-card design system, with a green "copied" state and light-mode support

Files changed

• script.js — updated showResult() markup, added copyResultUrl() and fallbackCopy()
• style.css — updated .result-url to flex layout, added .copy-url-btn styles

Testing

• Verified copy works on a safe result, danger result, and error result (URL present in all three)
• Verified button reverts to "Copy" after timeout and on repeat clicks
• Checked light mode and dark mode rendering

[GeethaBurigalla]

PR is ready for review!!