Function renderFigureCitation

PROTOTYPE: Render function for figure citations

This is a prototype implementation that follows the same pattern as renderEquationCitation() but adapted for figures. The full implementation will require:

1. FigureCache integration to fetch figure data
2. Figure preview popover (showing image instead of math)
3. Settings for figure citation format
4. Cross-file figure resolution
5. Continuous citation support for figures

Usage:

• User writes: $\ref{fig:3.1}$ in markdown
• Parser detects "fig:" prefix
• Routes to this function instead of equation renderer
• Displays as: (fig:3.1) or customizable format