feat: Codex-style project workspace and parallel chats

[Guo-Yixin]

Summary

• Add Codex-style collapsible project sidebar, project and chat lifecycle, per-chat drafts, model selection, and delete confirmation.
• Add durable PostgreSQL/SQLite run queue, bounded concurrent workers, resumable SSE events, cancellation, stale-run recovery, and isolated repository workspaces with optional OpenSandbox runtime.
• Add implementation plan and backend/frontend regression coverage.

Verification

• Python suite: 165 passed, 1 warning.
• UI Vitest: 10 passed.
• UI production build and Python compileall passed.
• Live PostgreSQL API smoke checks passed; real model generation was not run.