Resolve conflicts for staging→main sync (PR #170):
- postcss/vite/package.json: keep staging's Tailwind 4 setup
- model-selection/selection: keep main's model divider feature
- api-key-settings: keep main's BYOK tooltip + labeled buttons
- stream_v2.go: drop duplicated llmProvider block
- footer/index.tsx: import Button from @heroui/react (no @heroui/button dep)
- regenerated package-lock.json; build + lint pass

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EvF4FfL7fkGBbvhHLuAWN3