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

src/popup/views/transactionDetail.js

@@ -27,7 +27,7 @@ const { nativeCurrencyByChainId } = require("../../shared/networks");
 const { formatEther, formatUnits } = require("ethers");
 const { log, debugFetch } = require("../../shared/log");
 const { storedSettingIsOn } = require("../../shared/storedSettings");
-const { decodeCalldata } = require("./approval");
+const { decodeCalldata } = require("../../shared/calldataDecoder");
 
 /**
  * Determine a human-readable transaction type string from tx fields.