fix: the ETH line reads unknown at an address no refresh has read (closes #736)

An address's ETH balance is now null until a refresh has read it. That covers
a new address, a new wallet, addresses a wallet scan finds, and every address
after a network switch.

Its line reads `quantity unknown`, Home's balance `unknown (ETH)`, and Send
`Current balance: unknown (ETH)`. Max says the balance is unknown. Send is no
longer refused as zero. The address has no total, and DeleteAddress warns that
it may hold a balance.

A refresh also deletes `balanceNotFetched` only when it read every address in
the stored record.

Model: opus-5-5

src/shared/chainSwitchFields.js

@@ -68,10 +68,11 @@ function applyChainSwitchFields(s, newNetworkId) {
 
     // Clear per-address balances and token balances so stale data
     // from the previous chain is never displayed while the first
-    // refresh on the new chain is in flight.
+    // refresh on the new chain is in flight. A balance of null is one no
+    // refresh has read: it reads as unknown until one has, not as 0.0000.
     for (const wallet of s.wallets || []) {
         for (const addr of wallet.addresses || []) {
-            addr.balance = "0";
+            addr.balance = null;
             addr.tokenBalances = [];
         }
     }
@@ -108,7 +109,7 @@ function copyChainSwitchFields(s, switched) {
     for (const wallet of s.wallets) {
         for (const addr of wallet.addresses) {
             const got = stored.get(addr.address);
-            addr.balance = got ? got.balance : "0";
+            addr.balance = got ? got.balance : null;
             addr.tokenBalances = got ? got.tokenBalances : [];
         }
     }