feat: Flush notebook to disk on restart

[mchav]

📝 Summary

Mis-timed restarts don't save the notebook to disk properly and result in data loss. The PR flushes the notebook to disk before initiating a restart.

📋 Pre-Review Checklist

• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
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✅ Merge Checklist

• I have read the contributor guidelines.
• Documentation has been updated where applicable, including docstrings for API changes.
• Tests have been added for the changes made.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Jun 26, 2026 4:29pm

[cubic-dev-ai]

No issues found across 2 files

Architecture diagram

sequenceDiagram
    participant User as User
    participant UI as useRestartKernel
    participant Save as useSaveNotebook
    participant Persist as Notebook Persistence
    participant WS as WebSocket
    participant Kernel as Kernel

    User->>UI: Click restart kernel
    UI->>UI: Open confirm dialog
    UI->>User: Show updated description
    User->>UI: Confirm restart
    UI->>Save: saveIfNotebookIsPersistent()
    Note right of Save: Checks if notebook is a persistent file

    alt Notebook is persistent file
        Save->>Persist: Save notebook to disk
        Persist-->>Save: Save result
    else Save fails (caught)
        Save-->>UI: Error thrown
        UI->>UI: Logger.warn() – non‑fatal
    end

    UI->>WS: setConnection(CLOSING)
    UI->>Kernel: sendRestart()
    Kernel-->>UI: Acknowledged
    UI->>UI: reloadSafe() → page reload

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_{Re-trigger cubic}