Enhancement: Analyte Comparison slide type in PPT/QMD export

[Gero1999]

Description

Add an "Analyte Comparison" slide type under the Summary Slides section in the slide customisation tree. This slide type generates mean plots faceted by both DOSETRT and PARAM, allowing users to compare analytes side-by-side within each treatment group.

The new section should:

• Appear as a subsection under "Summary Slides" in the slide customisation modal
• Be selected by default when exporting slides
• Generate mean concentration-time plots using exploration_meanplot() with facet_by = c("DOSETRT", "PARAM")
• Be included in both PPTX and QMD export formats

Definition of Done

• "Analyte Comparison" subsection added to .build_slide_types() in zip.R
• Analyte comparison mean plots generated in get_dose_esc_results() in zip-utils.R
• Analyte comparison slides rendered in PPTX export (officer-utils.R)
• Analyte comparison slides rendered in QMD export (quarto-utils.R)
• Unit tests cover the new slide type
• NEWS.md updated