Correct the sign of context effects

[strengejacke]

No description provided.

[gemini-code-assist]

Code Review

This pull request updates the modelbased package to version 0.14.0.12, primarily changing the contrast calculation logic in get_contexteffects.R from diff(x) to revpairwise and removing the hypothesis column. These changes necessitated updates to the test suite and the 'practical context effect' vignette to account for flipped signs in the results. Feedback focuses on ensuring the sign of the contrast remains consistent with the documented 'Between - Within' definition, as revpairwise may have inverted it. Additionally, several inconsistencies in the vignette's text and statistical values need to be addressed to match the updated calculations.