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/account-state-manager.ts

@@ -37,33 +37,37 @@ export function snapshotAccount(accountId: string): void {
   const identityState = useIdentityStore.getState();
   const vacationState = useVacationStore.getState();
 
+  // Copy the captured collections so the snapshot is decoupled from the live
+  // store: a later in-place mutation (e.g. an array push/splice, or stamping
+  // fields onto a shared email object) must not retroactively corrupt a
+  // snapshot taken earlier.
   cache.set(accountId, {
     email: {
-      emails: emailState.emails,
-      mailboxes: emailState.mailboxes,
+      emails: [...emailState.emails],
+      mailboxes: [...emailState.mailboxes],
       selectedEmail: emailState.selectedEmail,
       selectedMailbox: emailState.selectedMailbox,
       searchQuery: emailState.searchQuery,
-      quota: emailState.quota,
+      quota: emailState.quota ? { ...emailState.quota } : emailState.quota,
     },
     contact: {
-      contacts: contactState.contacts,
-      addressBooks: contactState.addressBooks,
+      contacts: [...contactState.contacts],
+      addressBooks: [...contactState.addressBooks],
       supportsSync: contactState.supportsSync,
     },
     calendar: {
-      calendars: calendarState.calendars,
-      events: calendarState.events,
-      selectedCalendarIds: calendarState.selectedCalendarIds,
+      calendars: [...calendarState.calendars],
+      events: [...calendarState.events],
+      selectedCalendarIds: [...calendarState.selectedCalendarIds],
       viewMode: calendarState.viewMode,
       supportsCalendar: calendarState.supportsCalendar,
     },
     filter: {
-      rules: filterState.rules,
+      rules: [...filterState.rules],
       isSupported: filterState.isSupported,
     },
     identity: {
-      identities: identityState.identities,
+      identities: [...identityState.identities],
       preferredPrimaryId: identityState.preferredPrimaryId,
     },
     vacation: {
@@ -73,11 +77,22 @@ export function snapshotAccount(accountId: string): void {
   });
 }
 
-/** Restore cached store states for the given account. Returns false if no cache exists. */
+/**
+ * Restore cached store states for the given account. Returns false if no cache
+ * exists.
+ *
+ * The snapshot only captures a subset of each store's fields (the loaded data),
+ * so we reset every store to its baseline first. Without this, fields outside
+ * the captured subset (e.g. email selection, loading flags, tag counts) would
+ * carry over from whatever account was active, leaking state across accounts.
+ * `setState` merges, so the captured fields are then layered back on top.
+ */
 export function restoreAccount(accountId: string): boolean {
   const snapshot = cache.get(accountId);
   if (!snapshot) return false;
 
+  clearAllStores();
+
   useEmailStore.setState(snapshot.email);
   useContactStore.setState(snapshot.contact);
   useCalendarStore.setState(snapshot.calendar);