Improve settings app providers UX and API key status display
- Check API keys via os.getenv() to reflect true runtime availability
(shell env + .env file), not just journal config
- Show "(system)" indicator for keys available from system environment
- Add save feedback checkmarks to provider/tier detail task overrides
- Rename tiers to Best/Average/Fast, section to "Detailed Tasks"
- Pre-select built-in tier defaults when they differ from user default
- Make nav sticky below facet bar, improve active item visibility
- Consolidate API_KEY_ENV_VARS constant, remove unused CSS classes
- Fix updateEnvStatus() missing argument bug
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>