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 and each
recipient to Etherscan. 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. Each gated request reads its setting from storage
before it asks and when the answer lands, and drops a late answer. An open popup
takes another page's change and draws its screens again. While "Use Blockscout"
is off, no address keeps an untracked token, and no screen names a token from
Blockscout's report. ENS lookups ask only the RPC endpoint.

Model: opus-5-5 (implementation); fable-5-1 (prose)

README.md

@@ -1184,8 +1184,8 @@ screens show the line:
 - TxApproval and SignApproval
 - WaitTx, SuccessTx and ErrorTx, which follow TxApproval
 
-Where none of the sources answers, the amount is not formatted: the line reads
-`5000000000 base units (decimals unknown)`. A guessed scale is not an
+Where none of the sources answers, the amount is not formatted. The amount line
+then reads `5000000000 base units (decimals unknown)`. A guessed scale is not an
 approximation but a different number — 1,000 units of a 6-decimal token
 formatted at 18 decimals reads `0.000000001` — on the screen whose only job is
 to state what is being authorized. Both amount paths of that screen take this