Add first version of the CLAUDE.md

CLAUDE.md

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+# Remark42 Development Guidelines
+
+## Build/Test/Lint Commands
+- **Backend**:
+  - Run server: `make rundev`
+  - Build: `make backend`
+  - Race test: `make race_test`
+- **Backend Testing**:
+  - Run all tests: `cd backend/app && go test -timeout=60s -count 1 ./...`
+  - Run single test: `cd backend/app && go test -run TestName ./path/to/package`
+- **Frontend**:
+  - Development: `cd frontend && pnpm dev:app`
+  - Tests: `cd frontend && pnpm test`
+- **Lint**:
+  - Backend: `golangci-lint run`
+  - Frontend: `cd frontend && pnpm lint`
+
+## Code Style
+- **Backend**: Formatting with golangci-lint, strict error handling
+- **Frontend**: TypeScript with ESLint, Stylelint and Prettier
+- **Imports**: Group stdlib, external packages, then internal packages
+- **CSS**: CSS Modules for new components (`component.module.css`)
+
+## Key Backend Packages
+- **Web/API**: `github.com/go-chi/chi/v5`, `github.com/go-pkgz/rest`
+- **Auth**: `github.com/go-pkgz/auth/v2`
+- **Logging**: `github.com/go-pkgz/lgr`
+- **Testing**: `github.com/stretchr/testify`
+- **Notifications**: `github.com/go-pkgz/notify`
+
+## Repository Structure
+- Backend: Go server using BoltDB for storage
+- Frontend: Preact/Redux-based UI with iframe embedding