Cross-Dataset comparison view (multi-dataset regional plot) per Liu Fei's design

[boxiangliu]

From Liu Fei's mockup (#23): a 'Cross-Dataset' mode that compares a locus across datasets/tissues, alongside the single 'Locus View'. Deferred from the first frontend PR (#27, Locus View only). Builds on the /api/gene/{key}/regional backend (#25).