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/auth/session-cookie.ts

@@ -1,2 +1,7 @@
 export const SESSION_COOKIE = 'jmap_session';
 export const SESSION_COOKIE_MAX_AGE = 30 * 24 * 60 * 60;
+
+/** Get the cookie name for a given account slot (0-4). Slot 0 uses the legacy name. */
+export function sessionCookieName(slot: number): string {
+  return slot === 0 ? SESSION_COOKIE : `${SESSION_COOKIE}_${slot}`;
+}