fix: a chain switch gives trackedTokens its own copy of the network's list (closes #719)

After a switch, `trackedTokens` and that network's list in
`networkTrackedTokens` were one array. So a page's next add or removal also
changed `networkTrackedTokens`, and the save wrote that page's older copy over
the stored one. Another page's add on the previous network was lost, and its
removal was undone.

`applyChainSwitchFields()` now copies the incoming network's list. An add or
removal on the active network changes only `trackedTokens`.

Model: opus-5-5

src/shared/chainSwitchFields.js

@@ -42,11 +42,15 @@ function applyChainSwitchFields(s, newNetworkId) {
     // Tracked tokens are kept per network the same way, because a contract
     // address names a token on one network only. s.trackedTokens stays the
     // active network's list, so nothing that reads it changes: the outgoing
-    // list is put away under its network, and the incoming network's list, or
-    // an empty one, takes its place.
+    // list is put away under its network, and a copy of the incoming network's
+    // list, or an empty list, takes its place.
+    //
+    // It is a copy because a save writes a changed s.networkTrackedTokens whole,
+    // over any list another page stored. An add or removal on the active network
+    // must change only s.trackedTokens.
     if (!s.networkTrackedTokens) s.networkTrackedTokens = {};
     s.networkTrackedTokens[s.networkId] = s.trackedTokens;
-    s.trackedTokens = s.networkTrackedTokens[net.id] || [];
+    s.trackedTokens = structuredClone(s.networkTrackedTokens[net.id] || []);
 
     s.networkId = net.id;
     s.rpcUrl = remembered.rpcUrl || net.defaultRpcUrl;