feat(wallet): add Copy address button to QRCode component

[Petah1]

Closes #561

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Summary

• Adds a "Copy address" button beneath the QR code canvas
• Uses navigator.clipboard.writeText when available; falls back to execCommand('copy') via a temporary <textarea> for unsupported browsers
• Button label temporarily changes to "Copied!" for 2 seconds then reverts, with timer cleanup on unmount to prevent memory leaks

Test plan

• Open the wallet QR code modal and confirm the "Copy address" button renders below the QR image
• Click the button in a modern browser — verify the wallet address is in the clipboard and the label changes to "Copied!" then reverts after 2 seconds
• Test in a browser with navigator.clipboard disabled/unavailable — verify the fallback selects the address and the copy still succeeds
• Rapidly click the button multiple times — confirm no timer stacking or duplicate "Copied!" states
• Unmount the component while the "Copied!" label is showing — confirm no console warnings about state updates on unmounted components

[vercel]

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