Refresh Theme Studio branding and streamline import workflow

[kkugot]

Summary

• refresh the app branding from Theme Editor to Theme Studio, bump the package to 1.2.0, and add the README demo video
• replace the legacy installer with a shared import/export script and add the JSON import guide for bringing local OpenCode themes into the studio
• polish the editor controls and preview details, including preset/search controls, copy buttons, mixer undo flows, and terminal titlebar alignment

Verification

• npm run lint
• npm run build