fix: restore tour Next button (#294)

[milanofthe]

Fixes #294.

Problem

In the guided tours the Next button stops working from step 2 onward — it renders but does nothing. Reported for both the Start and Modeling tours.

cause

The builders rawStep, blockStep and the result step of actionStep always set the onNextClick key on the popover object — when no hook is passed, as onNextClick: undefined.

An explicit onNextClick: undefined therefore clobbers driver.js's built-in Next handler. The click then falls through to an unregistered nextClick event and nothing happens.

Step 1 works because intro/floatingStep never sets the onNextClick key — which is exactly why the failure starts at step 2.

Fix

New prunePopover() helper in builder.ts that strips keys with undefined values before the popover is handed to driver.js, applied to the three affected builders.