Disable theme transitions during theme swaps

[KayleeWilliams]

Summary

Disable CSS transitions only while Orca flips the document theme class so light/dark swaps happen instantly instead of fading different UI elements at different speeds.

Changes

• Added a shared renderer document theme helper that resolves system, dark, and light preferences.
• Temporarily applies a root transition-suppression class during theme swaps and removes it after two animation frames.
• Wired startup, settings, and system theme change handling through the shared helper.
• Added focused unit coverage for theme resolution, root class toggling, and transition suppression cleanup.

Validation

• pnpm test src/renderer/src/lib/document-theme.test.ts
• pnpm run tc:web
• pnpm exec oxlint src/renderer/src/lib/document-theme.ts src/renderer/src/lib/document-theme.test.ts src/renderer/src/main.tsx src/renderer/src/App.tsx src/renderer/src/components/settings/Settings.tsx
• git diff --check