Auto-withdraw runs with the app closed; fix the dead Back on the legacy step

The service worker can't watch a relay, so it doesn't try: the app
resolves the destination to something concrete (ark address, LNURL
endpoint, or on-chain script) and mirrors it beside the keys the
background NWC payer already uses. On a push wake-up — which is exactly
when money has just landed — the worker re-checks the same gate and
sends, writing the outcome back into the mirror for the app to absorb on
next open. A partial on-chain forward still waits for the app, since
carving an exact vtxo needs more round trips than a worker is promised.

writeBgMirror now carries forward the NWC connections and offer when
another caller refreshes the mirror, instead of blanking them.

And onboarding's legacy step loses its Back: the wallet exists by then,
so returning to the nostr question just bounced straight forward again
and the link looked broken. Its copy no longer promises a rename either.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Wvj4aoeMZwA7VUGdJP9Swi

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