fix: read USD prices from Chainlink price feeds through the RPC endpoint (closes #525)

CoinDesk now refuses requests without an API key, so mainnet showed no USD prices. Mainnet prices now come from Chainlink's USD price feeds, read with eth_call through the user's RPC endpoint; no other service is asked. The bundled list pairs each of 53 tokens with its feed by contract address, and ETH and WETH use ETH/USD. The popup reads the feeds of ETH and the tokens the user holds or tracks in one batched request, at most every 5 minutes. A reading older than its heartbeat plus 30 minutes, or not positive, is no price. A token with no feed shows no USD value. CoinDesk is gone.

RULES.md still names the CoinDesk price API.

Model: opus-5-5

docs/README.md

@@ -70,9 +70,9 @@ analytics, use a portfolio tracker. The wallet is not the place for any of that.
 ## How It Works
 
 AutistMask is a browser extension that runs entirely in your browser. It does
-not have a backend server. Out of the box it sends your data only to the
-Ethereum RPC endpoint. It also asks the CoinDesk price API for prices, sending
-token symbols and nothing else.
+not have a backend server. Out of the box it contacts only the Ethereum RPC
+endpoint. USD prices are read from Chainlink price feed contracts through that
+endpoint.
 
 Blockscout and Etherscan are sent none of your data until you turn on a setting
 for each. The settings are "Use Blockscout" and "Check recipients on Etherscan".
@@ -89,10 +89,16 @@ verify confirmations. The default is a free public RPC endpoint. You can change
 this in Settings to any Ethereum JSON-RPC endpoint, including your own local
 node.
 
+On mainnet, AutistMask also reads USD prices through this endpoint. The prices
+come from Chainlink's price feed contracts. A token with no Chainlink feed shows
+no USD value.
+
 When it is contacted:
 
 - on every balance refresh (every 10 seconds while the popup is open, every 60
   seconds in the background)
+- on mainnet, to read USD prices, at most every 5 minutes while the popup is
+  open
 - when you review a send to an ENS name
 - when a send is prepared and broadcast
 - while a pending transaction is polled for its receipt
@@ -131,19 +137,6 @@ sends none of your data.
 What gets sent: your Ethereum addresses (to look up balances and transactions)
 and the hash of each transaction you open.
 
-**CoinDesk CADLI price API** (`data-api.coindesk.com`)
-
-Used to fetch current USD prices for ETH and the top 25 tokens. Prices are
-cached for 5 minutes. No API key is required. This endpoint is not
-user-configurable, and it is not contacted at all while you are on a testnet,
-where no USD values are shown.
-
-When it is contacted: while the popup is open, at most once every 5 minutes.
-
-What gets sent: token symbol names (e.g. "ETH", "USDC"). No addresses, no
-balances, no identifying information. As with any request, CoinDesk sees your IP
-address.
-
 **Etherscan address labels** (`etherscan.io`; `sepolia.etherscan.io` on Sepolia)
 
 While "Check recipients on Etherscan" is on, AutistMask fetches the recipient's