Show dataset volume alongside compliance coverage in faculty/sub-dataverse charts

[pedrohserrano]

Feedback 20260507 - (I. Bleylevens): the current charts show compliance coverage, but dataset volume is only visible on hover. We should make it easier to see whether high/low compliance groups represent a small or large number of datasets.

• Add visible dataset counts to the faculty/sub-dataverse chart labels or bars.
• Explore a percentage-based view alongside absolute counts.
• Consider a visual encoding for volume, e.g. bubble size, secondary label, or dual metric card.