Feature/dea functions

[DanielGarbozo]

feat: add DEA workflow, example datasets, and improve function documentation

New vignette:

• DEA_workflow.Rmd: full RNA-seq DEA workflow with pre-rendered figures
• prerender_figures.R: script to regenerate vignette figures
• vignettes/figures/: pre-rendered PNG plots (power, PCA, UMAP, tSNE, Volcano, VSB)

Documentation improvements (all functions):

• Add @return, @Seealso, @note to all DEA functions
• Add @references to nice_UMAP, nice_tSNE.
• Update @param label_var in nice_Volcano and @param symbol in nice_VSB
  to mention get_annotations() for symbol-based labeling
• Add note for nice_VSB_DESeq2 alternative in DEA vignette

Bug fixes:

• get_annotations: fix rowNames → row.names in write.csv
• save_results: add underscore separator and include l2fc/cutoff_alpha
  values in output filenames for traceability
• power_analysis: return $plot slot in output list; plot built
  conditionally only when plot = TRUE

Other:

• Update DESCRIPTION: LazyDataCompression xz, DESeq2 to Suggests
• Update .Rbuildignore: exclude vignettes/prerender_figures.R
• Update NAMESPACE and man/