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/networkEndpoints.test.js

@@ -47,6 +47,7 @@ function loadModuleWith(persisted) {
     return {
         mod: require("../src/shared/state"),
         chainSwitch: require("../src/shared/chainSwitch"),
+        storage,
         written: () => storage.read("autistmask"),
     };
 }
@@ -105,6 +106,27 @@ describe("a custom endpoint survives a chain switch", () => {
         expect(mod.state.rpcUrl).toBe(CUSTOM_RPC);
     });
 
+    // Another page saves the user's own node for Ethereum Mainnet after this
+    // page's last save. This page's switch keeps that node for Ethereum
+    // Mainnet.
+    test("an endpoint another page saved for the network being left is remembered", async () => {
+        const { mod, chainSwitch, storage, written } = loadModuleWith({
+            wallets: walletFixture(),
+            networkId: "mainnet",
+            rpcUrl: MAINNET.defaultRpcUrl,
+            blockscoutUrl: MAINNET.defaultBlockscoutUrl,
+            networkEndpoints: {},
+        });
+        await mod.loadState();
+        storage.write("autistmask", { ...written(), rpcUrl: CUSTOM_RPC });
+
+        await chainSwitch.onChainSwitch("sepolia");
+        expect(written().networkEndpoints.mainnet.rpcUrl).toBe(CUSTOM_RPC);
+
+        await chainSwitch.onChainSwitch("mainnet");
+        expect(mod.state.rpcUrl).toBe(CUSTOM_RPC);
+    });
+
     test("the remembered endpoints survive an extension restart", async () => {
         const first = loadModuleWith({
             wallets: walletFixture(),