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

README.md

@@ -264,6 +264,8 @@ The shared modules, each in one line:
   messaging, windows and the lock.
 - `buildInfo.js`: the version, license, author, commit and date compiled into
   About.
+- `calldataDecoder.js`: the ERC-20 `approve()` and `transfer()` decoder, which
+  hands other calldata to `uniswap.js`. Popup only.
 - `chainSwitch.js`: the popup's network switch.
 - `chainSwitchFields.js`: the fields a network switch changes.
 - `constants.js`: `DEBUG`, the default endpoints, the ERC-20 ABI and the burn
@@ -1379,7 +1381,7 @@ approval screens show a red warning.
 ### Transaction decoding
 
 TxApproval and TransactionDetail try to decode calldata into a summary
-(`decodeCalldata()` in `src/popup/views/approval.js`). SuccessTx repeats
+(`decodeCalldata()` in `src/shared/calldataDecoder.js`). SuccessTx repeats
 TxApproval's summary. Decoding is a display aid. When it fails, the raw calldata
 is shown in full. No contract gets any treatment beyond a legible confirmation
 screen.