fix: a network switch in Settings leaves no balance from the previous network stored (closes #653)

The switch is now made on the stored record that its save has just read. The switched fields are then copied into the page's state. A save writes only the fields that the page changed since its last save. A switch made on the page's state alone mishandled what another page stored since then:

- A balance from the previous network stayed under the new network.
- A token tracked on the previous network moved to the new network.
- An endpoint URL saved for the previous network was replaced by the page's older one.

Tests store each of these before the switch and check the stored record after it.

Model: opus-5-5

tests/trackedTokensPerNetwork.test.js

@@ -177,6 +177,25 @@ test("a token added by contract address on Sepolia is tracked on Sepolia only",
     expect(onSepolia).not.toContain("DAI");
 });
 
+// Another popup page tracks DAI on Ethereum Mainnet after this page's last
+// save. This page's switch in Settings keeps DAI, tracked on Ethereum Mainnet.
+test("a token another page tracked stays on its network through a switch in Settings", async () => {
+    const boot = await bootPopup(unversionedValidProfile());
+    expect(boot.pageErrors).toEqual([]);
+    boot.storage.write("autistmask", {
+        ...boot.storage.read("autistmask"),
+        trackedTokens: [MAINNET_TOKEN],
+    });
+
+    await boot.click("btn-settings");
+    await chooseNetwork(boot, "sepolia");
+
+    const stored = boot.storage.read("autistmask");
+    expect(stored.networkId).toBe("sepolia");
+    expect(stored.trackedTokens).toEqual([]);
+    expect(stored.networkTrackedTokens.mainnet).toEqual([MAINNET_TOKEN]);
+});
+
 describe("Back after a switch from the Settings gear", () => {
     test("SettingsAddToken no longer offers the common tokens", async () => {
         const boot = await bootOnAddToken("mainnet");