fix: Blockscout and Etherscan lookups are off until the user turns them on (closes #668)

Out of the box the wallet sent the user's addresses to Blockscout for history
and token lookups. It sent each recipient to Etherscan for its label. Two
settings, "Use Blockscout" and "Check recipients on Etherscan", now gate those
requests. Both are off in a new install and offered unticked on Welcome and in
Settings. While one is off, the screens that relied on it say so in a plain
line. The balance refresh and the Etherscan check read the stored setting. An
open popup takes a change another page saved and draws its screens again. While
"Use Blockscout" is off, no address keeps an untracked token. ENS lookups ask
only the RPC endpoint.

Model: opus-5-5

src/shared/transactions.js

@@ -235,12 +235,17 @@ function mergeTransactions(txs, tokenTransfers) {
 // `chainId` is the chain id of the network `blockscoutUrl` serves. Every entry
 // carries it, and a native entry is labelled with that network's
 // `nativeCurrency` from networks.js (`ETH`, `SepoliaETH`).
+//
+// `useBlockscout` is the user's "Use Blockscout" setting. While it is off,
+// Blockscout is not asked and the answer is an empty list.
 async function fetchRecentTransactions(
     address,
     blockscoutUrl,
+    useBlockscout,
     chainId,
     count = 25,
 ) {
+    if (!useBlockscout) return [];
     log.debugf("fetchRecentTransactions", address);
     const addrLower = normalizeAddress(address);