feat: implement Zustand state management

[vreshch]

Summary

Implements Zustand stores for centralized state management across the renderer process.

Changes

New Store Files

• src/renderer/stores/auth.store.ts - Authentication state management
• src/renderer/stores/chat.store.ts - Chat session and streaming state
• src/renderer/stores/conversation.store.ts - Conversation history management
• src/renderer/stores/models.store.ts - Model providers and configuration
• src/renderer/stores/tools.store.ts - Tools and enabled/disabled state
• src/renderer/stores/workspace.store.ts - Workspace management
• src/renderer/stores/types.ts - Shared store types
• src/renderer/stores/index.ts - Store initialization and exports

Refactored Components

• src/renderer/app/App.tsx - Use store initialization
• src/renderer/layouts/components/WorkspaceSwitcher.tsx - Use workspace store
• src/renderer/pages/tools/ToolsPage.tsx - Use tools store
• src/renderer/pages/tools/components/ToolCard.tsx - Use tools store
• src/renderer/pages/workspaces/page.tsx - Use workspace store

Other Changes

• package.json - Add zustand dependency
• src/shared/types/conversation.types.ts - Add ConversationMessage type
• src/main/index.ts - Minor updates
• src/main/services/conversation.store.service.ts - Minor updates