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

A save writes only the fields that differ from what the page last saved. When
the page had last saved a balance of "0", the switch's cleared "0" was no
change. So a balance that the background or another page stored since then
stayed under the new network.

The switch's save now also resets these fields in the stored record it reads:

- every address's balance and token balances;
- the time of the last balance refresh.

Every other field merges as before.

Model: opus-5-5

src/shared/chainSwitch.js

@@ -48,7 +48,7 @@ function clearPopupCaches() {
 // waits for the lock (followStoredSettings() in src/popup/index.js): that
 // follow runs first, and this switch is made after it.
 //
-// The save writes only the fields that this page changed since its last save.
+// The save writes only the fields that differ from what this page last saved.
 // When this page last saved a balance of "0", clearing that balance is no
 // change. So the switch also resets these fields in the stored record:
 //

src/shared/state.js

@@ -592,9 +592,9 @@ function reportSaveFailure(err) {
 // `beforeWrite`, when given, is called inside the lock with the stored record
 // as just read, before this page's state is compared against it. It may change
 // `state`, and this save writes that change. It may also change the stored
-// record: a field that this page did not change is written as `beforeWrite`
-// left it. When it returns false, nothing is written; any other return lets the
-// save go ahead.
+// record. A field that this page did not change is then written as
+// `beforeWrite` left it. When it returns false, nothing is written; any other
+// return lets the save go ahead.
 function saveState(beforeWrite) {
     const turn = withLock("autistmask", () => saveStateOnce(beforeWrite));
     // Every failed save is reported, whether or not the caller awaited this

tests/trackedTokensPerNetwork.test.js

@@ -199,7 +199,7 @@ test("a token another page tracked before a switch in Settings stays in trackedT
 
 // The contract lookup on SettingsAddToken answers on Ethereum Mainnet while the
 // switch to Sepolia waits for the lock. The page adds the token to Ethereum
-// Mainnet's list, and the switch then keeps that list for Ethereum Mainnet.
+// Mainnet's list. The switch then keeps that list for Ethereum Mainnet.
 test("a token added while a switch in Settings waits for the lock stays tracked on Ethereum Mainnet", async () => {
     const boot = await bootOnAddToken("mainnet");
     expect(boot.pageErrors).toEqual([]);