fix: a tracked token reads quantity unknown at an address no refresh has read (closes #720)

A new address, a new wallet and a network switch each leave tracked tokens
with no balance entry, so their lines read 0.0000 before any refresh had read
them.

Each of these now sets `balanceNotFetched` on the tracked tokens, and a refresh
already clears it. Their lines read `quantity unknown` until then. Addresses
that the wallet scan finds get the same. A refresh that did not read every
address the page now holds leaves the field set.

Model: opus-5-5

tests/popupFollowsNetworkSwitch.test.js

@@ -245,7 +245,9 @@ test("a switch back in Settings after a follow whose write failed is stored", as
     const stored = page.storage.read("autistmask");
     expect(stored.networkId).toBe("mainnet");
     expect(stored.rpcUrl).toBe(NETWORKS.mainnet.defaultRpcUrl);
-    expect(stored.trackedTokens).toEqual([MAINNET_TOKEN]);
+    expect(stored.trackedTokens).toEqual([
+        { ...MAINNET_TOKEN, balanceNotFetched: true },
+    ]);
     expect(page.text("flash-msg")).toBe("Switched to Ethereum Mainnet.");
     const told = globalThis.chrome.runtime.sendMessage.mock.calls
         .map(([message]) => message)