Add comprehensive E2E testing plan and project guidelines

- Create CLAUDE.md with project commands and code style guidelines
- Add detailed E2E testing plan in prds/add-e2e-tests.md covering:
  - Priority-based testing strategy for critical user workflows
  - Implementation constraints requiring only test ID additions
  - Mandatory test validation workflow with npm run test:e2e
  - Test coverage for expense splitting, balance tracking, and group management
- Remove CRUSH.md (replaced by CLAUDE.md)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,44 @@
+# CLAUDE.md
+
+## Build, Lint, and Test Commands
+- **Build the project**: `npm run build`
+- **Start the project**: `npm run start`
+- **Run the project in development**: `npm run dev`
+- **Lint the project**: `npm run lint`
+- **Check types**: `npm run check-types`
+- **Format code**: `npm run prettier`
+- **Test the project**: `npm run test`
+- **Run a single test**: Use Jest's `-t` option, e.g., `npm run test -- -t 'test name'`
+
+## Code Style Guidelines
+
+### Import Conventions
+- Use `import` statements for importing modules.
+- Organize imports using **prettier-plugin-organize-imports**.
+- Import globals from libraries before local modules.
+
+### Formatting
+- Use **Prettier** for code formatting.
+- Adhere to a line width of 80 characters where possible.
+- Use 2 spaces for indentation.
+
+### Types
+- Utilize TypeScript for static typing throughout the codebase.
+- Define interfaces and types for complex objects.
+
+### Naming Conventions
+- Use camelCase for variable and function names.
+- Use PascalCase for component and type/interface names.
+
+### Error Handling
+- Use `try...catch` blocks for async functions.
+- Handle errors gracefully and log them where required.
+
+### Miscellaneous
+- Ensure all new components are functional components.
+- Prefer arrow functions for component definition.
+- Use hooks like `useEffect` and `useState` for managing component state.
+
+---
+
+**Note**: Please follow these guidelines to maintain consistency and quality within the codebase.