feat: add Zustand state management

- Add Zustand (v5.0.6) for lightweight state management
- Create sessionStore for managing session-related state
- Create agentStore for managing agent runs with intelligent polling
- Eliminate prop drilling across component tree
- Add comprehensive documentation for store usage

package.json

@@ -57,7 +57,8 @@
     "remark-gfm": "^4.0.0",
     "tailwind-merge": "^2.6.0",
     "tailwindcss": "^4.1.8",
-    "zod": "^3.24.1"
+    "zod": "^3.24.1",
+    "zustand": "^5.0.6"
   },
   "devDependencies": {
     "@tauri-apps/cli": "^2",