Add unattended update-only startup mode

[patchzyy]

What changed

• add a --updateonly startup mode
• update an existing Retro Rewind installation without opening the main window
• automatically install a Wheel Wizard update when one is available
• exit after the update check instead of relaunching the normal UI

Why

Wheel Wizard's existing startup flow could launch Retro Rewind from scripts, but updating still required interactive confirmation. This adds the smallest unattended path needed by keyboardless launcher setups such as Steam and ES-DE.

Retro Rewind is only updated when it is already installed; this mode does not perform a fresh installation.

Closes #293

Validation

• dotnet build WheelWizard.sln --configuration Release
• dotnet test WheelWizard.sln --configuration Release --no-build (174 passed)

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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• Commit unit tests in branch codex/issue-293-updateonly

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[patchzyy]

GitHub closes an open pull request when its head branch is renamed. The branch is now issue-293-update-only; the replacement draft PR is #303.