Refactor: consolidate fragmented Zustand stores using slice pattern

[sudorishabh]

Problem

The React package currently uses Zustand with multiple tiny stores (e.g. sidebarStore, pinnedMessageStore, searchMessageStore, fileStore, etc.). Many of these are single-boolean + setter stores and require importing/subscribing to many stores in central components.

This “store fragmentation” makes cross-panel logic harder (e.g. opening one panel should close others) and increases maintenance overhead.

Proposed improvement

Adopt a slice / consolidated-store approach in @embeddedchat/react:

• Group all sidebar/panel visibility flags into a single store (useChatLayoutStore) with an openExclusivePanel(panelKey) API.
• Group chat “meta/data” (starting with starred messages) into a single store (useChatDataStore).
• Consolidate auth modal state (isLoginIn, isLoginModalOpen, isTotpModalOpen) into one auth store (useAuthStore).

Acceptance criteria

• No remaining imports/usages of removed stores/hook.
• Sidebar panel toggling remains exclusive and behavior-preserving.
• Auth modals and login loading state work as before.
• Starred messages continue to load and render correctly.