Restore Hub workspace browser styling

[youngbeom-shin]

Summary

• Migrate the Hub workspace browser polish from PR Polish Hub workspace browser UI #65 into the current Vite React frontend.
• Add folder/file icons, the illustrated empty preview state, scroll-aware panels, and line-numbered code preview rendering.
• Update the Hub workspace browser styles to match the dark split-pane file browser treatment from the legacy frontend work.

Validation

• PATH=/Users/shenyongfan/.nvm/versions/node/v22.13.0/bin:$PATH pnpm --dir web/app format
• PATH=/Users/shenyongfan/.nvm/versions/node/v22.13.0/bin:$PATH pnpm --dir web/app typecheck
• PATH=/Users/shenyongfan/.nvm/versions/node/v22.13.0/bin:$PATH pnpm --dir web/app lint (passes with existing warnings outside this change)

[jialudev]

LGTM @RussellLuo