feat: deploy interactive CMS demo to GitHub Pages

[owjs3901]

Summary

• add a browser-only mock API for the example app's plugin screens
• make static routes and assets work under the GitHub Pages project path
• deploy the 22-route example frontend through the official Pages workflow
• link the public demo from the README

Verification

• \�un x tsc --noEmit\ in \packages/app\
• \�un run lint\ (passes with one existing auth warning)
• \VITE_YEOLLIN_DEMO=true VITE_YEOLLIN_BASE_PATH=/yeollin-cms cargo run -p yeollin-cli -- build --skip-backend\
• static export: 22 routes, 0 skipped