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/shared/amountDisplay.js

@@ -2,7 +2,7 @@
 // the one exception to it, in one place. Three call sites had grown their own
 // copy of the truncation — the history and balance lists
 // (`src/shared/transactions.js`), the approval screen's ERC-20 amount line
-// (`src/popup/views/approval.js`) and its Uniswap swap detail lines
+// (`src/shared/calldataDecoder.js`) and its Uniswap swap detail lines
 // (`src/shared/uniswap.js`) — and a fix applied to one of them left the other
 // two showing a different number for the same value.
 //