Add LangGraph agent with SSE streaming and notifications UI

[yawbtng]

Summary

• LangGraph Agent Backend: Custom StateGraph agent with intent routing, tool execution, and human-in-the-loop approval flow for email/calendar actions
• SSE Streaming API: Real-time chat endpoint with event types (status, token, tool_result, approval_required, done, error)
• Google Workspace Integration: GWS CLI runner for Gmail, Calendar, Docs, and Drive tools
• Heartbeat Service: Background loop for proactive nudges (upcoming meetings, overdue tasks, urgent emails)
• Notifications UI: Frontend panel with real-time polling and notification context provider
• Supporting Infrastructure: Session/task management APIs, in-memory store, Pydantic settings config, convenience start script

New Backend Modules

• backend/app/agent/ — StateGraph definition, nodes, prompts, state, tools (GWS, Exa, memory, notifications, data)
• backend/app/api/ — Chat (SSE), sessions, tasks, health, notifications endpoints
• backend/app/core/ — GWS CLI runner, SSE helpers
• backend/app/services/ — Heartbeat service, in-memory store
• backend/app/config.py — Pydantic settings from .env

Frontend Changes

• frontend/src/components/Notifications/ — NotificationsPanel + NotificationCard
• frontend/src/contexts/NotificationsContext.tsx — Global notification state
• Updated layout and sidebar to integrate notifications

Test plan

• Backend starts without errors: cd backend && python app/main.py
• Health endpoint responds: GET /health
• Chat SSE endpoint streams events: POST /api/chat
• Notifications panel renders in frontend sidebar
• Human-in-the-loop approval flow blocks email/calendar actions