fix: merge wallets by identity, not whole-field, in saveState() (closes #304)

backgroundRefresh() mutates state.wallets in place (addr.balance/ensName/
tokenBalances via refreshBalances()), so a whole-field diff on `wallets`
marked the entire array "changed" the moment any balance moved and wrote
back background's own copy -- loaded before its multi-second network
round trip -- clobbering a wallet another page added, or resurrecting one
another page deleted, in that window. That is DoD item 2 on the issue,
still unmet by the prior whole-field merge.

`wallets` is now merged structurally: by wallet identity (xpub for
hd/xprv wallets, address for key wallets, both already enforced unique),
then by address identity within each wallet. A leaf background actually
changed applies on top of storage's current copy; membership added or
removed by another page applies independently, since it no longer
collides with `wallets` as a single field. Every other persisted field
stays a whole-field diff -- no code path mutates them the way
backgroundRefresh() mutates wallets, so there is no matching defect to
fix there.

tests/stateMerge.test.js

@@ -212,3 +212,80 @@ describe("the approval-window reproduction", () => {
         ]);
     });
 });
+
+// backgroundRefresh() (src/background/index.js) loads state, spends seconds
+// on network I/O in refreshBalances() (src/shared/balances.js) mutating
+// addr.balance/ensName/tokenBalances IN PLACE on the wallets it already
+// knew about, then saves. Precondition 2 on the issue: that refresh window
+// overlapping a membership change (add or delete) on another page must not
+// clobber or resurrect a wallet — a whole-field diff on `wallets` failed
+// this, because "background changed a balance" and "another page changed
+// membership" collided as the same field.
+describe("background refresh racing a wallet added on another page", () => {
+    test("the wallet added elsewhere survives background's stale balance save", async () => {
+        const storage = makeStorage();
+        await storage.set({ autistmask: { wallets: [W1] } });
+
+        // "background": loads first, and its save is the one that lands
+        // last, modeling the multi-second network round trip in between.
+        const background = loadPage(storage);
+        await background.state.loadState();
+        background.state.state.wallets[0].addresses[0].balance = "1.2345";
+
+        // A second page, loaded after, adds a wallet while background's
+        // refresh is still in flight.
+        const popup = loadPage(storage);
+        await popup.state.loadState();
+        popup.state.state.wallets.push(W2);
+        popup.state.state.hasWallet = true;
+        await popup.state.saveState();
+        expect(
+            (await storage.get("autistmask")).autistmask.wallets,
+        ).toHaveLength(2);
+
+        // background's save lands last, carrying only its balance update.
+        await background.state.saveState();
+
+        const persisted = (await storage.get("autistmask")).autistmask;
+        expect(persisted.wallets.map((w) => w.name)).toEqual([
+            "Wallet 1",
+            "Wallet 2",
+        ]);
+        expect(persisted.wallets.map((w) => w.encryptedSecret)).toEqual([
+            "secret-one",
+            "secret-two",
+        ]);
+        // The balance update itself must not be lost either — this is a
+        // merge, not deletion-always-wins.
+        expect(persisted.wallets[0].addresses[0].balance).toBe("1.2345");
+    });
+});
+
+describe("background refresh racing a wallet deleted on another page", () => {
+    test("the wallet deleted elsewhere stays deleted after background's stale balance save", async () => {
+        const storage = makeStorage();
+        await storage.set({ autistmask: { wallets: [W1, W2] } });
+
+        const background = loadPage(storage);
+        await background.state.loadState();
+        background.state.state.wallets[0].addresses[0].balance = "1.2345";
+
+        // A second page deletes Wallet 2 while background's refresh is in
+        // flight — the same splice deleteWallet.js's removeWalletFromState()
+        // does.
+        const popup = loadPage(storage);
+        await popup.state.loadState();
+        popup.state.state.wallets.splice(1, 1);
+        popup.state.state.hasWallet = popup.state.state.wallets.length > 0;
+        await popup.state.saveState();
+        expect(
+            (await storage.get("autistmask")).autistmask.wallets,
+        ).toHaveLength(1);
+
+        await background.state.saveState();
+
+        const persisted = (await storage.get("autistmask")).autistmask;
+        expect(persisted.wallets.map((w) => w.name)).toEqual(["Wallet 1"]);
+        expect(persisted.wallets[0].addresses[0].balance).toBe("1.2345");
+    });
+});