chore: remove dead exports and share copied view helpers (closes #168)

AddressDetail and AddressToken defined their own isoDate() and
timeAgo(), hiding the shared pair in helpers.js, so a fix there would
not have reached them. The copies were identical and are deleted;
blockieHtml() and tokenLabel(), each defined twice, move to helpers.js.
A new test shows the history rows and the transaction detail view
write the time with the shared pair.

Deleted as never called: explorerLink(), ETHEREUM_SEPOLIA_CHAIN_ID,
getWalletValue() and getTotalValue() with their tests. Home's "Total:"
is the active address's total, as README.md already says.
addressColor() and etherscanAddressUrl() are no longer exported.

Model: opus-5-5

README.md

@@ -842,9 +842,11 @@ The same data must be formatted identically everywhere it appears. Token and ETH
 amounts are displayed with exactly 4 decimal places (e.g. "1.0500 ETH", "17.1900
 USDT") in balance lists, transaction lists, send confirmations, and approval
 screens. Timestamps include both an ISO datetime and a humanized relative age
-wherever shown. If a formatting rule applies in one place, it applies in every
-place. Users should never see the same value rendered differently on two
-screens.
+wherever shown, written by `isoDate()` and `timeAgo()` in
+`src/popup/views/helpers.js` on every screen; the ISO datetime is in UTC when
+the UTC Timestamps setting is on. If a formatting rule applies in one place, it
+applies in every place. Users should never see the same value rendered
+differently on two screens.
 
 The native token's label is a network's `nativeCurrency` in
 `src/shared/networks.js`: `ETH` on mainnet, `SepoliaETH` on Sepolia. The