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/popup/views/addWallet.js

@@ -190,7 +190,7 @@ async function importMnemonic(ctx) {
             {
                 address: firstAddress,
                 index: 0,
-                balance: "0.0000",
+                balance: null,
                 tokenBalances: [],
             },
         ],
@@ -218,7 +218,7 @@ async function importMnemonic(ctx) {
         wallet.addresses = scan.addresses.map((a) => ({
             address: a.address,
             index: a.index,
-            balance: "0.0000",
+            balance: null,
             tokenBalances: [],
         }));
         wallet.nextIndex = scan.nextIndex;
@@ -267,7 +267,7 @@ async function importPrivateKey(ctx) {
         type: "key",
         name: "Wallet " + walletNum,
         encryptedSecret: encrypted,
-        addresses: [{ address: addr, balance: "0.0000", tokenBalances: [] }],
+        addresses: [{ address: addr, balance: null, tokenBalances: [] }],
     });
     markTrackedTokensNotFetched();
     state.hasWallet = true;
@@ -328,7 +328,7 @@ async function importXprvKey(ctx) {
             {
                 address: firstAddress,
                 index: 0,
-                balance: "0.0000",
+                balance: null,
                 tokenBalances: [],
             },
         ],
@@ -356,7 +356,7 @@ async function importXprvKey(ctx) {
         wallet.addresses = scan.addresses.map((a) => ({
             address: a.address,
             index: a.index,
-            balance: "0.0000",
+            balance: null,
             tokenBalances: [],
         }));
         wallet.nextIndex = scan.nextIndex;