feat: implement account switcher component and state management

- Add AccountSwitcher component for managing user accounts with UI for switching, adding, and logging out.
- Create account state manager to handle snapshots of account-specific states for efficient switching.
- Introduce utility functions for account management, including ID generation and avatar color assignment.
- Implement Zustand store for account management, supporting addition, removal, and state retrieval of accounts.

lib/account-state-manager.ts

@@ -0,0 +1,116 @@
+/**
+ * Manages per-account state snapshots for fast switching.
+ * When user switches from Account A → B, we snapshot A's store state
+ * into memory, clear stores, then restore B's cached state.
+ */
+
+import { useEmailStore } from '@/stores/email-store';
+import { useContactStore } from '@/stores/contact-store';
+import { useCalendarStore } from '@/stores/calendar-store';
+import { useFilterStore } from '@/stores/filter-store';
+import { useIdentityStore } from '@/stores/identity-store';
+import { useVacationStore } from '@/stores/vacation-store';
+
+// Minimal snapshot shapes — we only capture what we need
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+type StoreSnapshot = Record<string, any>;
+
+interface AccountSnapshot {
+  email: StoreSnapshot;
+  contact: StoreSnapshot;
+  calendar: StoreSnapshot;
+  filter: StoreSnapshot;
+  identity: StoreSnapshot;
+  vacation: StoreSnapshot;
+}
+
+const cache = new Map<string, AccountSnapshot>();
+
+/** Capture current store states for the given account */
+export function snapshotAccount(accountId: string): void {
+  const emailState = useEmailStore.getState();
+  const contactState = useContactStore.getState();
+  const calendarState = useCalendarStore.getState();
+  const filterState = useFilterStore.getState();
+  const identityState = useIdentityStore.getState();
+  const vacationState = useVacationStore.getState();
+
+  cache.set(accountId, {
+    email: {
+      emails: emailState.emails,
+      mailboxes: emailState.mailboxes,
+      selectedEmail: emailState.selectedEmail,
+      selectedMailbox: emailState.selectedMailbox,
+      searchQuery: emailState.searchQuery,
+      quota: emailState.quota,
+    },
+    contact: {
+      contacts: contactState.contacts,
+      addressBooks: contactState.addressBooks,
+      supportsSync: contactState.supportsSync,
+    },
+    calendar: {
+      calendars: calendarState.calendars,
+      events: calendarState.events,
+      selectedCalendarIds: calendarState.selectedCalendarIds,
+      viewMode: calendarState.viewMode,
+      supportsCalendar: calendarState.supportsCalendar,
+    },
+    filter: {
+      rules: filterState.rules,
+      isSupported: filterState.isSupported,
+    },
+    identity: {
+      identities: identityState.identities,
+      preferredPrimaryId: identityState.preferredPrimaryId,
+    },
+    vacation: {
+      isEnabled: vacationState.isEnabled,
+      isSupported: vacationState.isSupported,
+    },
+  });
+}
+
+/** Restore cached store states for the given account. Returns false if no cache exists. */
+export function restoreAccount(accountId: string): boolean {
+  const snapshot = cache.get(accountId);
+  if (!snapshot) return false;
+
+  useEmailStore.setState(snapshot.email);
+  useContactStore.setState(snapshot.contact);
+  useCalendarStore.setState(snapshot.calendar);
+  useFilterStore.setState(snapshot.filter);
+  useIdentityStore.setState(snapshot.identity);
+  useVacationStore.setState(snapshot.vacation);
+
+  return true;
+}
+
+/** Clear all stores (used before restoring a different account) */
+export function clearAllStores(): void {
+  useEmailStore.setState({
+    emails: [],
+    mailboxes: [],
+    selectedEmail: null,
+    selectedMailbox: '',
+    isLoading: false,
+    error: null,
+    searchQuery: '',
+    quota: null,
+  });
+  useIdentityStore.getState().clearIdentities();
+  useContactStore.getState().clearContacts();
+  useVacationStore.getState().clearState();
+  useCalendarStore.getState().clearState();
+  useFilterStore.getState().clearState();
+}
+
+/** Evict cached state for one account */
+export function evictAccount(accountId: string): void {
+  cache.delete(accountId);
+}
+
+/** Evict all cached states */
+export function evictAll(): void {
+  cache.clear();
+}