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

tests/balanceRefreshNetworkSwitch.test.js

@@ -70,7 +70,7 @@ async function chooseNetwork(page, networkId) {
 function expectNothingStored(page) {
     const stored = page.storage.read("autistmask");
     expect(stored.networkId).toBe("sepolia");
-    expect(stored.wallets[0].addresses[0].balance).toBe("0");
+    expect(stored.wallets[0].addresses[0].balance).toBeNull();
     expect(stored.wallets[0].addresses[0].tokenBalances).toEqual([]);
     expect(stored.lastBalanceRefresh).toBe(0);
 }
@@ -86,13 +86,13 @@ test("a refresh that returns after a switch in Settings writes nothing, and the
     await chooseNetwork(page, "sepolia");
     await refresh.finish();
 
-    expect(state.wallets[0].addresses[0].balance).toBe("0");
+    expect(state.wallets[0].addresses[0].balance).toBeNull();
     expect(state.wallets[0].addresses[0].tokenBalances).toEqual([]);
     expectNothingStored(page);
 
     await page.click("btn-settings-back");
     expect(page.visibleViews()).toEqual(["main"]);
-    expect(page.text("total-value")).toBe("0.0000 SepoliaETH");
+    expect(page.text("total-value")).toBe("unknown (SepoliaETH)");
 
     await page.tick();
 
@@ -138,7 +138,7 @@ test("a refresh whose save is reading the stored record when the user switches i
     await page.settle();
 
     expect(state.networkId).toBe("sepolia");
-    expect(state.wallets[0].addresses[0].balance).toBe("0");
+    expect(state.wallets[0].addresses[0].balance).toBeNull();
     expect(state.wallets[0].addresses[0].tokenBalances).toEqual([]);
     expectNothingStored(page);
 });
@@ -166,11 +166,11 @@ test("a refresh that returns after the background switched the network writes no
 });
 
 // The background's refresh saves Ethereum Mainnet's balances while the switch
-// in Settings waits for the lock. This page last saved a balance of "0" and no
+// in Settings waits for the lock. This page last saved a balance of null and no
 // token balances, the values that the switch sets.
 test("a background refresh saved while a switch in Settings waits for the lock leaves none of its balances stored", async () => {
     const profile = unversionedValidProfile();
-    profile.wallets[0].addresses[0].balance = "0";
+    profile.wallets[0].addresses[0].balance = null;
     const page = await bootPopup(profile);
     expect(page.pageErrors).toEqual([]);
     const background = require("../src/background/state");