Add Copy URL button to the result card

[NaitikVerma6776]

Summary

• Adds a Copy URL button next to the scanned URL in the result card so users can copy long URLs without selecting text
• Shows a brief Copied! confirmation for 2 seconds after a successful copy
• Styles the button to match the existing design system (card border, accent-cyan hover)

Closes #179

Test plan

• Scan a URL and confirm the result card shows a Copy URL button beside the URL
• Click Copy URL and verify the URL is on the clipboard
• Confirm the button label changes to Copied! for ~2 seconds, then reverts
• Check hover styles in both dark and light themes

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