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)

docs/README.md

@@ -295,9 +295,13 @@ The confirmation screen shows:
   estimate, and below it the larger amount reserved until it confirms. The
   reserve is what the network requires up front and what the balance check gates
   on; the refund of the difference is why the two differ
-- **Warnings** if the recipient is a contract, a burn address, one of your own
-  addresses, on the bundled scam-address list, or, while "Check recipients on
-  Etherscan" is on, labelled as a phisher on Etherscan
+- **Warnings** if the recipient is:
+    - a contract
+    - a burn address
+    - one of your own addresses
+    - on the bundled scam-address list
+    - while "Check recipients on Etherscan" is on, labelled as a phisher on
+      Etherscan
 
 After reviewing, enter your password and click "Sign & Send". The transaction
 will be broadcast to the network and you will see a waiting screen with a timer.