fix: make clipboard fallback dialog-safe

[chen-ran]

Summary

• keep the legacy clipboard fallback inside an active dialog or alert dialog so focus traps do not steal its selection
• restore focus and remove the temporary textarea after copying

Why

The existing fallback mounts its temporary textarea under document.body. In a focus-trapped dialog, focus is immediately redirected back into the dialog, so copying can fail over plain HTTP when navigator.clipboard is unavailable.

Verification

• UI type check passes
• production build passes
• ESLint passes
• UI contract guard passes