feat: move the stratigraphic layers to step 2 (6.2)

[lachlangrose]

Summary

• Remove the stratigraphic layers widget from step 1. Step 1 has the bounding box, the CRS and the DEM.
• Add the widget to step 2 in a "Source layers" tab, next to a "Column" tab, so the page is less tall.
• The first picker follows the contacts source. With "Calculate from geology polygons" it shows polygon layers and uses the geology roles. With "Use a contacts layer" it shows line and point layers and uses basal_contacts. Each source keeps its own selection.
• Update the "step 1" text and the docs. Keep the stratigraphic_layers_widget settings key.

Stacked on #137 (6.1). Implements 6.2 of the usability plan.

Test plan

• Unit tests pass (233)
• Manual check in QGIS: the picker filter and the kept selection for each source
• Qt test of the picker filter (not added yet)

🤖 Generated with Claude Code