fix: manage missing sketch folder

[dido18]

Motivation

The HTTP panics if an app does not have a sketch folder in the list of libraries (and possibly other endpoints).

Change description

• make the field MainSketchPath private
• add the GetSketchPath() (*paths.Path, bool) method to retrieve the sketch path
• fix the panic in other parts of the code
• add unit tests

Additional Notes

Reviewer checklist

• PR addresses a single concern.
• PR title and description are properly filled.
• Changes will be merged in main.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.