feat: add Electron desktop client prototype

[cwang0810]

Summary

• add apps/desktop Electron + Vite + TypeScript desktop client scaffold
• implement the latest OpenTalking desktop prototype UI with home, realtime chat, video generation, avatar/voice, model packages, works, tools, settings, and compatible WebUI entry
• add managed backend/profile IPC, loopback proxy plumbing, .otpkg model package management, QuickTalk package template, ffmpeg extra-resource handling, and electron-builder macOS/Windows packaging config
• add Chinese desktop client prototype docs and standalone HTML prototype

Validation

• cd apps/desktop && npm run build
• manually launched npx electron . and checked home, model package, realtime chat, and video generation pages

Notes

• Persona Package changes were split into feat: add Persona Package support #87.
• ffmpeg/ffprobe binaries, packaged app artifacts, node_modules, and other generated outputs are intentionally excluded by apps/desktop/.gitignore.
• QuickTalk local package is staged as a separate .otpkg flow; P1 keeps heavyweight runtime/model assets outside the main installer.