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/approvalDisplayFloor.test.js

@@ -13,7 +13,7 @@
 // asserted on `rawValue` alongside the displayed line.
 //
 // Both amount paths of that screen are covered: the ERC-20 line decoded by
-// `src/popup/views/approval.js`, and the swap's `Amount` and `Min. received`
+// `src/shared/calldataDecoder.js`, and the swap's `Amount` and `Min. received`
 // lines decoded by `src/shared/uniswap.js`.
 
 globalThis.chrome = {
@@ -23,7 +23,7 @@ globalThis.chrome = {
 const { AbiCoder, Interface } = require("ethers");
 const { ERC20_ABI } = require("../src/shared/constants");
 const { state } = require("../src/shared/state");
-const { decodeCalldata } = require("../src/popup/views/approval");
+const { decodeCalldata } = require("../src/shared/calldataDecoder");
 const uniswap = require("../src/shared/uniswap");
 const {
     truncateAmount,