fix: history rows show the ISO datetime and the age (closes #760)

Home, AddressDetail and AddressToken showed only the relative age, with
the datetime as a tooltip. Each row now shows the datetime and then the
age in parentheses, on a line of its own, through transactionTimeHtml()
in helpers.js, which TransactionDetail also uses. The direction label
moves to its own right-aligned line below. README.md and docs/README.md
describe the new row.

Model: opus-5-5

README.md

@@ -563,9 +563,10 @@ of a wallet is a block:
 
 "Recent Transactions" lists up to 25 transactions across every address of every
 wallet, deduplicated by hash and filtered (see Scam protection). Each row has
-three lines:
+four lines:
 
-- the age and the direction;
+- the ISO datetime and the relative age;
+- the direction, on the right;
 - the counterparty's colour dot and label, with the amount on the right;
 - the counterparty's full address, as plain text.
 
@@ -1185,8 +1186,7 @@ popup closes.
 These helpers write one kind of value on every screen that shows it:
 
 - `formatUsd()` in `src/shared/prices.js`: USD values;
-- `isoDate()` and `timeAgo()` in `src/popup/views/helpers.js`: transaction
-  times;
+- `transactionTimeHtml()` in `src/popup/views/helpers.js`: transaction times;
 - `renderAddressHtml()` in the same file: most addresses.
 
 Most screens show token and ETH amounts with four decimal places. History rows
@@ -1274,9 +1274,8 @@ A native amount is labelled with the native currency of the transaction's chain:
 `src/shared/networks.js`). The balance lists and Send use the active network's.
 The decoded swap lines call the native currency `ETH` on both networks.
 
-TransactionDetail shows the time as an ISO datetime and a relative age. A
-history row shows the age, with the datetime as its tooltip. With "UTC
-Timestamps" on, the datetime is in UTC.
+TransactionDetail and every history row show the time as an ISO datetime and
+then a relative age. With "UTC Timestamps" on, the datetime is in UTC.
 
 Every nonzero holding is stored exactly. A token holding below 0.000001 is left
 out of: