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

@@ -273,9 +273,8 @@ screen. Tokens can also be added from Settings, under "Tracked Tokens".
 ## Sending
 
 1. Click "Send" from the home screen or an address detail view.
-2. Select what to send: ETH, or a token you track that this address holds. While
-   "Use Blockscout" is on, Send also offers other tokens with a balance on this
-   address that survive the spam filters.
+2. Select what to send (ETH, or any ERC-20 token with a balance on this address
+   that survives the spam filters).
 3. Enter the recipient address or ENS name (e.g. `vitalik.eth`). AutistMask does
    not look up a name resolved off-chain. For such a name, enter the address
    instead.
@@ -296,8 +295,8 @@ The confirmation screen shows:
   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
+  addresses, on the bundled scam-address list, or 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.
@@ -308,9 +307,8 @@ screen with the transaction hash and an Etherscan link.
 
 You can also send from a token-specific view: on the address detail screen,
 click any token balance row. This opens a focused view showing only that token's
-transactions. While "Use Blockscout" is off, the view says instead that no
-transaction history is shown. Clicking "Send" from here pre-selects and locks
-the token so you cannot accidentally switch to a different one.
+transactions. Clicking "Send" from here pre-selects and locks the token so you
+cannot accidentally switch to a different one.
 
 ## Receiving
 
@@ -384,12 +382,11 @@ balances unless they are on the bundled known-token list or you added them
 yourself. Legitimate tokens have substantial holder counts; scam tokens deployed
 for address poisoning typically have zero. When the explorer reports no holder
 count for a token, or reports something other than a whole number in plain
-digits (such as "1,000"), the count is unknown. While "Use Blockscout" is off,
-no holder count is known. An unknown count does not hide a token from your
-transaction history or the send token list, and it does not get a token into
-your balances either: such a token is listed only if it is on the bundled
-known-token list or you added it yourself. The screen you reach by clicking a
-token balance shows a "Holders:" line only when the count is known.
+digits (such as "1,000"), the count is unknown. An unknown count does not hide a
+token from your transaction history or the send token list, and it does not get
+a token into your balances either: such a token is listed only if it is on the
+bundled known-token list or you added it yourself. The screen you reach by
+clicking a token balance shows a "Holders:" line only when the count is known.
 
 **Fraud contract blocklist.** When AutistMask detects a fraudulent transfer, it
 adds the contract address to a local blocklist. Future transactions from that