Preserve partial generation on disconnect

[abi]

Summary

• Preserve in-progress generation output when the WebSocket disconnects unexpectedly.
• Mark unfinished variants as errored and return the UI to the code review state instead of resetting.
• Show a connection-loss message that explains partial output was preserved.
• Tighten the generation WebSocket payload typing in the touched file.

Walkthrough

disconnect_preserves_partial_landing_page.mp4

Connection lost with partial landing page preserved

Testing

• Passed: pnpm exec eslint src/App.tsx src/generateCode.ts --ext ts,tsx --report-unused-disable-directives --max-warnings 0
• Passed: pnpm exec tsc --noEmit
• Manual: fake WebSocket backend streamed a partial SaaS landing page and then dropped the transport; the UI preserved the partial landing page and showed the connection-loss error.
• Warning: full pnpm lint still fails on existing repository lint debt outside this change.

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