Map themes: group layer visibility ignored & only first theme selectable

[dominik-warch]

Two issues with QGIS Desktop map themes in QWC2:

• Group layer visibility is ignored. When switching themes, only individual layer visibility is applied. Group layer visibility from the QGIS project is not respected. Groups always appear disabled regardless of how the theme is configured in QGIS.
• Theme selection always applies the first theme. No matter which theme is clicked in the theme switcher, only the topmost/first theme gets applied.

Stock viewer, QWC2 Build 2026.04.14