Fix desktop welcome clipping on fullscreen resize / 修复桌面端全屏后欢迎页裁剪

[SivanCola]

Summary

• Track the live viewport height so the desktop shell follows macOS fullscreen and resize updates.
• Let the empty transcript scroll instead of clipping the welcome actions, and reset empty-state scroll position.
• Keep the native window background synced with the active theme to avoid exposing a black shell during repaint gaps.

Fixes #4494

Verification

• npm run check:css
• npx tsx src/__tests__/typography-overflow-contract.test.ts
• npm run build
• Browser preview checked with ?platform=darwin&mock=fresh at normal and short viewport heights.

[chatgpt-codex-connector]

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