Cleanup: Remove B&W Voorstellen (VoorstellenPanel) remnants from codebase

[rubenvdlinde]

Context

The B&W Voorstellen widget was removed from the case detail page in the feature/dashboard branch because B&W voorstellen is a separate zaaktype (or managed in DecideDesk), not a widget on a case detail page. This was likely a spec error.

What was done

• Removed the <VoorstellenPanel> widget from CaseDetail.vue
• Removed the import and component registration

What still needs cleanup

• src/views/cases/components/VoorstellenPanel.vue — component file can be deleted
• src/views/voorstellen/ — entire directory with VoorstelDetail.vue, VoorstelList.vue, components/ParafeerActionBar.vue etc. — review if these should be removed or moved to DecideDesk
• Router definitions for voorstellen routes — clean up
• voorstel object type registration in store (currently errors with "not registered") — remove or move to DecideDesk
• Check if any seed data references voorstellen

Why

B&W Voorstellen is not a case widget — it's either a standalone zaaktype or functionality that belongs in DecideDesk. The current implementation was likely generated from an incorrect spec.