move the wallet's chain source out of env and into the ui

WALLET_ESPLORA_URL was the one wallet setting an owner actually has to change — off a
public explorer that rate-limits and sees every address, onto their own indexer — and it
was the one they could only change with a shell and a restart. It now lives in
service_connections under 'esplora' and is edited at Wallet -> Settings -> Chain source,
probed against /blocks/tip/height before it is stored.

The URL joins the backend fingerprint, so re-pointing rebuilds every on-chain backend and
drops the gap-limit scan taken through the old endpoint. /_health probes what the wallets
actually use rather than the built-in default, and /_officer/config no longer reports a URL
it cannot know.

Also two receive-screen defects the Blockstream 429 exposed: a query error rendered as
"No address available", and the "new address" button called refetch() on the ?peek=true
query, so it re-fetched the same address instead of advancing the index.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/databases/officer_db/src/queries/service-connections.ts

@@ -13,7 +13,7 @@ import { encryptSecret, decryptSecret } from '../crypto';
 // moment someone forgot to strip it.
 
 /** The services that keep a connection here. Extending it is a one-line change, not a migration. */
-export type ServiceName = 'transmission' | 'slskd';
+export type ServiceName = 'transmission' | 'slskd' | 'esplora';
 
 export type ServiceConnection = {
   id: number;