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

CoinDesk now refuses every request without an API key, so mainnet showed no
USD prices. On mainnet the wallet now reads latestRoundData() from Chainlink's
USD price feeds through the configured RPC endpoint. ETH and the held or
tracked tokens with a feed are read as one batched request, at most every 5
minutes. src/shared/tokenList.js lists the feeds of 53 bundled tokens, keyed by
contract address, and the ETH/USD feed. A reading older than its feed's
heartbeat plus 30 minutes, or not positive, is no price. A failed read leaves
no price and is logged. getPrice() takes a contract address instead of a
symbol. The CoinDesk request is removed.

Model: opus-5-5

src/shared/prices.js

@@ -28,7 +28,8 @@ let lastFetchedAt = 0;
 // logged, and that token then has no price.
 //
 // `signal`, passed by the popup, is aborted when the popup closes. A read that
-// fails after that was cancelled by the closing and is not logged.
+// fails once `signal` is aborted is not logged, because closing the popup
+// cancelled it.
 async function refreshPrices(signal) {
     const { state, currentNetwork } = require("./state");
     if (currentNetwork().id !== "mainnet") {

tests/e2e/network.js

@@ -253,8 +253,8 @@ const SELECTOR_NAME = "0x06fdde03";
 const SELECTOR_BALANCE_OF = "0x70a08231";
 const SELECTOR_LATEST_ROUND_DATA = "0xfeaf968c";
 
-// A price feed's latestRoundData() is answered on any contract with five zero
-// words. That is a round with a price of zero, which the wallet counts as no
+// On any contract, a latestRoundData() call is answered with five zero words.
+// That is a round with a price of zero, which the wallet counts as no
 // price. So no screen shows a USD value beside the amounts the tests compare.
 //
 // Every other eth_call answers with a zero word except decimals(), symbol(),