docs: add CLAUDE.md with project guidance for Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Expense tracker app from Mosh's [Claude Code course](https://codewithmosh.com/p/claude-code). It intentionally has bugs, poor UI, and messy code — all fixed throughout the course.
+
+## Commands
+
+- `npm run dev` — Start Vite dev server (http://localhost:5173)
+- `npm run build` — Production build
+- `npm run lint` — ESLint
+
+## Architecture
+
+Single-component React 19 app with Vite 7. All state and UI live in `src/App.jsx` — no routing, no state library, no backend. Plain CSS styling via `App.css` and `index.css`.
+
+## Known Intentional Issues
+
+- Transaction amounts stored as strings, so `reduce()` concatenates instead of summing
+- "Freelance Work" is typed as "expense" instead of "income"