Apply taste polish to Electron rebuild

[w2968066]

Summary

• Applies the requested taste-skill pass to the Electron/FastAPI rebuild from PR 重构为 Electron + FastAPI 双端架构，加入 macOS/Windows 自动打包 #1.
• Adds a prominent VX contact badge to the desktop header so the rebuilt UI preserves the public contact treatment.
• Removes runtime Google Fonts import and uses native Windows/macOS system font stacks for a desktop app that starts consistently offline.

Verification

• npm run build