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 keeps its
warning.

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

Model: opus-5-5

tests/sendMax.test.js

@@ -432,6 +432,27 @@ describe("Max on an ETH send", () => {
         },
     );
 
+    test("says so when no refresh has read the ETH balance", async () => {
+        await refreshWith(BALANCE_WEI);
+        state.wallets[0].addresses[0].balance = null;
+        openSend();
+        await pressMax();
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe("This token's balance is unknown.");
+    });
+
+    test("fills nothing in and says nothing when a network switch cleared the balance while the fee was estimated", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        const release = holdFeeEstimate();
+        const pressed = pressMax();
+        state.wallets[0].addresses[0].balance = null;
+        release();
+        await pressed;
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe("");
+    });
+
     test("fills nothing in and says nothing once Send was left while the fee was estimated", async () => {
         // 0.0001 ETH, which does not cover the fee: a result that landed
         // would say so on whichever screen is shown.