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

@@ -2130,8 +2130,7 @@ view would leave a wallet one click from deletion.
     - Amount + symbol, or for a swap the approval screen showed no `Amount` for,
       the same as on WaitTx: `Unknown (could not be decoded)`, unless its
       `Token In` is `ETH (native)`
-    - The same line under the amount as on WaitTx, whenever WaitTx would show
+    - While "Use Blockscout" is off, the same line under the amount as on WaitTx
-      it.
     - To: color dot + full address + etherscan link, or for a contract creation
       the same sentence as on WaitTx
     - Error message (dashed border box)