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/resolveTokenSymbol.test.js

@@ -22,7 +22,7 @@ const { Interface } = require("ethers");
 const { ERC20_ABI } = require("../src/shared/constants");
 const { state } = require("../src/shared/state");
 const { resolveTokenSymbol } = require("../src/shared/approvalAmount");
-const { decodeCalldata } = require("../src/popup/views/approval");
+const { decodeCalldata } = require("../src/shared/calldataDecoder");
 
 const iface = new Interface(ERC20_ABI);