Fix Sign Message copy button in non-secure contexts

[dmnyc]

Problem

On the Sign Message tool, the Copy button does nothing when ThunderHub is accessed over a non-secure context — plain HTTP, or a LAN / .onion / Tailscale address. In those contexts navigator.clipboard is undefined, so the existing handler:

navigator.clipboard.writeText(signed).then(() => toast.success('Signature Copied'))

throws immediately, and because there is no .catch, it fails silently with no feedback. Since most self-hosted ThunderHub instances are reached over exactly these non-HTTPS addresses, the button is effectively broken for a large share of users.

Fix

Extract the copy handler and add a fallback: use the async Clipboard API when it is available in a secure context, otherwise fall back to a temporary <textarea> + document.execCommand('copy'). Both paths surface a success toast, and the handler is wrapped in try/catch so a failure shows an error toast instead of silently doing nothing.

Testing

• Built and ran locally against a live LND node.
• Signed a message and clicked Copy over a non-HTTPS address (previously a no-op) — the success toast now appears and the clipboard is populated.
• Verified the copied signature is complete and valid via lncli verifymessage (valid: true).