AI-powered expense creation via natural language

[pdhruv93]

Summary

• Add @opensplit/ai package — a chat agent that creates expenses from natural language using Vercel AI SDK (v6), OpenAI, and the existing MCP server
• Add a floating chat popover in the web app with markdown rendering and i18n support
• Add "Create with AI" button to the add expense form
• Add dotenv support to the MCP server so it reads .env for port/URL configuration

What's included

New package: packages/ai

• System prompt tailored for expense creation (handles equal splits and custom paid amounts)
• Connects to the MCP server over HTTP via @ai-sdk/mcp — no tool duplication, reuses all existing MCP tools
• createChatHandler() factory that the Next.js API route calls with MCP URL, user API key, and OpenAI key

Web app changes

• /api/chat route handler — authenticates via the user's cookie and proxies to the AI agent
• ChatPopover component — floating bottom-right chat panel with useChat hook, markdown rendering (react-markdown + @tailwindcss/typography), and streaming status
• "Create with AI" sparkles button on the expense form that opens the chat
• All new strings use next-intl (no hardcoded text)

Other changes

• MCP server now loads .env via dotenv (resolves relative to package dir, not cwd)
• MCP .env.example added
• Expenses ordered by createdAt desc (was date desc)
• Docker compose updated: web service gets OPENSPLIT_MCP_URL and OPENAI_API_KEY, depends on MCP
• README updated with AI package docs, screenshots, env var tables, and demo disclaimer