feat: unify environment scripts with start.sh

- Add unified start.sh script with subcommands (all/backend/frontend)
- Add process management (--stop, --status)
- Remove separate start_backend.sh and start_frontend.sh
- Update setup_dev_env.sh with pre-flight checks and --cpu-only/--skip-db options
- Update .env.example to remove sensitive data and add DIFY translation config
- Add .pid/ to .gitignore for process management

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

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

frontend/.env.example

@@ -1,4 +1,7 @@
+# Frontend Environment Configuration
+# Copy this file to .env.local and configure
+
 # Backend API URL
-# For WSL2, use the WSL2 IP address (get it with: hostname -I)
-# For native Linux/Mac, use http://localhost:8000
-VITE_API_URL=http://172.20.20.106:8000
+# For local development: http://localhost:8000
+# For WSL2: Use the WSL2 IP address (get with: hostname -I | awk '{print $1}')
+VITE_API_URL=http://localhost:8000