grass.jupyter: Add legend support to InteractiveMap (#5468)

[saurabh12nxf]

Added raster legend support to grass.jupyter.InteractiveMap as requested in #5468.

Changes

• Created legend.py module with parse_colors() and generate_legend_html()
• Added add_legend() method to InteractiveMap class
• Supports both folium and ipyleaflet backends
• Automatically detects categorical vs continuous rasters
• Professional CSS styling matching Leaflet native controls

Usage

import grass.jupyter as gj

m = gj.InteractiveMap()
m.add_raster("elevation")
m.add_legend("elevation", title="Elevation (m)")
m.show()

Note : Tested parse_colors() with r.colors.out (elevation, landclass96), verified HTML generation and legend attachment logic for both backends; full end-to-end testing not possible locally due to ReprojectionRenderer initialization issues in WSL/OSGeo4W (unrelated to legend code); requesting CI validation for complete integration testing.

[github-actions]

[ruff] _{reported by reviewdog 🐶}

Suggested change

[petrasovaa]

Please include one or more screenshots, thank you.