refactor: organize tests and extract formatting utilities

- Move test files to lib/__tests__/ directory
- Extract formatting utilities (formatVersion, formatFileSize, formatDateTime)
  from fs-utils.ts to new utils.ts module
- Add Jest test configuration and test scripts
- Update component imports to use new utils module
- Add CLAUDE.md documentation for project structure

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,100 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+FTDL (Resource Download Website) is a Next.js-based internal network APK resource download tool. It provides a web interface for searching and downloading Android APK files organized by projects and versions.
+
+## Development Commands
+
+```bash
+# Install dependencies
+npm install
+
+# Start development server
+npm run dev
+
+# Build for production
+npm run build
+
+# Start production server
+npm start
+
+# Run tests
+npm test
+npm run test:watch
+npm run test:coverage
+```
+
+## Environment Variables
+
+Required environment variables (create in `.env.local`):
+
+- `AUTH_USERNAME` - Login username (default: `admin`)
+- `AUTH_PASSWORD` - Login password (default: `changeme`)
+- `RESOURCE_PATH` - Path to APK storage directory
+
+## Project Structure
+
+```
+ftdl/
+├── app/                 # Next.js App Router
+│   ├── api/            # API routes
+│   ├── download/       # Main download page
+│   ├── login/          # Login page
+│   └── layout.tsx      # Root layout
+├── lib/                # Shared utilities and constants
+│   ├── __tests__/      # Test files for lib modules
+│   ├── constants.ts    # Environment variables and config
+│   ├── fs-utils.ts     # File system operations with security
+│   └── utils.ts        # Helper functions (formatting)
+├── public/             # Static assets
+├── middleware.ts       # Authentication middleware
+└── *.config.*          # Configuration files (Jest, Next.js, etc.)
+```
+
+## Architecture
+
+### Security Layer
+
+All file system operations go through `lib/fs-utils.ts` which implements `validatePath()` to prevent path traversal attacks. The middleware (middleware.ts) protects all routes except `/login` using a simple cookie-based authentication.
+
+### File System Abstraction
+
+The application expects this resource directory structure:
+
+```
+RESOURCE_PATH/
+├── PROJECT_NAME/
+│   ├── VERSION_NAME/
+│   │   └── apks/
+│   │       └── *.apk
+```
+
+- Version names use underscores (e.g., `0_42`) but display with dots (`0.42`)
+- APK filenames can contain commit IDs which are extracted via regex `([a-f0-9]{7,})` for filtering
+
+### API Routes
+
+Located in `app/api/`:
+- `/api/projects` - List all project directories
+- `/api/versions?project={name}` - List versions for a project
+- `/api/apks?project={name}&version={name}&commitId={id}` - List APK files with optional commit filtering
+- `/api/download?project={name}&version={name}&apk={name}` - Stream APK file for download
+- `/api/login` - Handle authentication (sets cookie)
+
+### Data Fetching
+
+Client-side uses SWR for data fetching in the download flow:
+1. User selects project → fetch projects
+2. User selects version → fetch versions for that project
+3. Optional commit filter → fetch filtered APKs
+4. Download button triggers streaming download via API
+
+### Key Files
+
+- `lib/fs-utils.ts` - Core file system utilities with path validation
+- `lib/constants.ts` - Environment variable defaults and cookie configuration
+- `middleware.ts` - Route protection via Next.js middleware
+- `app/download/page.tsx` - Main UI with search form and APK results