fix: improve dev mode UI and configuration
- Fix capitalization: "You've reached" -> "you've reached"
- Add tooltip to dev indicator explaining it shows dummy data
- Add DEV_MODE environment variable to .env.template with documentation
- Prepare configuration for preview build deployment
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>