refactor: move the calldata decoder to src/shared/calldataDecoder.js (closes #759)

decodeCalldata() and tokenAmountText() move out of the TxApproval view into
src/shared/calldataDecoder.js. TxApproval, TransactionDetail and the decoder
tests import them from there.

tokenSourcesByChainId() moves from the popup view helpers to
src/shared/state.js, beside currentNetwork(), so the decoder requires nothing
under src/popup/. It reads only the state singleton and the network table.

Model: opus-5-5

tests/uniswapRouterAddress.test.js

@@ -47,7 +47,7 @@ globalThis.chrome = {
 const { state } = require("../src/shared/state");
 const { fetchRecentTransactions } = require("../src/shared/transactions");
 const approval = require("../src/popup/views/approval");
-const { decodeCalldata } = approval;
+const { decodeCalldata } = require("../src/shared/calldataDecoder");
 const transactionDetail = require("../src/popup/views/transactionDetail");
 
 const MAINNET = "0x1";