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

src/shared/networks.js

@@ -85,12 +85,6 @@ function nativeCurrencyByChainId(chainId) {
     return network ? network.nativeCurrency : "ETH";
 }
 
-// Build a block explorer link for the given path type and value.
-// type: "address" | "tx" | "token" | "block"
-function explorerLink(network, type, value) {
-    return `${network.explorerUrl}/${type}/${value}`;
-}
-
 module.exports = {
     NETWORKS,
     SUPPORTED_CHAIN_IDS,
@@ -99,5 +93,4 @@ module.exports = {
     networkById,
     networkByChainId,
     nativeCurrencyByChainId,
-    explorerLink,
 };