Add VideoDB-themed dark mode

[drukpa1455]

Summary

• add persisted System, Light, and Dark appearance settings
• apply the current VideoDB palette through shared semantic tokens
• theme the main renderer, recording flows, settings, and floating widget
• sync Electron native appearance so every window follows the selected theme

Verification

• npm test
• npm run build
• visually checked the Electron renderer in light and dark modes

npm run typecheck still reports the two existing LiveAssistPanel.tsx ref nullability errors on lines 379 and 392.