fix: isolate per-account state snapshots from leakage and mutation

account-state-manager had two latent correctness issues:

1. Shared references: snapshotAccount stored the live store arrays/objects
   directly, so a later in-place mutation (array push/splice, or a shared
   email object being stamped) retroactively corrupted an earlier snapshot.
   Now copies the captured collections.

2. Incomplete restore: the snapshot only captures a subset of each store's
   fields, but restoreAccount applied it with a merge, leaving every other
   field (email selection, loading flags, tag counts, …) at the previously
   active account's values. It only worked because every caller happened to
   call clearAllStores() first. restoreAccount now resets the stores to
   baseline itself before layering the snapshot back on, so it is correct
   standalone and can't leak state across accounts.

Adds tests pinning the isolation guarantees.

lib/__tests__/account-state-manager.test.ts

@@ -58,7 +58,7 @@ describe('snapshotAccount / restoreAccount', () => {
     expect(useVacationStore.getState().isEnabled).toBe(true);
   });
 
-  it('CHARACTERISATION: only snapshotted fields are restored; others survive', () => {
+  it('resets fields outside the snapshot subset to their defaults (no cross-account leak)', () => {
     // isLoading is NOT part of the email snapshot subset.
     useEmailStore.setState({ selectedMailbox: 'a-in', isLoading: false });
     snapshotAccount('A');
@@ -66,11 +66,11 @@ describe('snapshotAccount / restoreAccount', () => {
 
     restoreAccount('A');
 
-    expect(useEmailStore.getState().selectedMailbox).toBe('a-in'); // restored
-    expect(useEmailStore.getState().isLoading).toBe(true); // NOT restored (merge)
+    expect(useEmailStore.getState().selectedMailbox).toBe('a-in'); // captured → restored
+    expect(useEmailStore.getState().isLoading).toBe(false); // uncaptured → reset, not leaked
   });
 
-  it('CHARACTERISATION: snapshot stores array references, not deep clones', () => {
+  it('decouples the snapshot from later in-place mutation of the source array', () => {
     const arr = [makeEmail({ id: '1' })];
     useEmailStore.setState({ emails: arr });
     snapshotAccount('A');
@@ -78,8 +78,8 @@ describe('snapshotAccount / restoreAccount', () => {
     useEmailStore.setState({ emails: [] });
 
     restoreAccount('A');
-    // The post-snapshot mutation leaked into the snapshot.
-    expect(useEmailStore.getState().emails.map((e) => e.id)).toEqual(['1', '2']);
+    // The post-snapshot mutation did NOT leak into the snapshot.
+    expect(useEmailStore.getState().emails.map((e) => e.id)).toEqual(['1']);
   });
 
   it('returns false and leaves stores untouched for an unknown account', () => {