fix(learning-path): add legend for connection lines (dotted vs solid, colors)

## Category
Learning Path Visualization

## Problem
The learning path visualization shows a legend for nodes but doesn't explain what the connection lines mean (dotted vs solid lines, different colors).

## Expected Behavior
A legend should explain all visual elements including:
- Dotted vs solid connection lines
- Different line colors and what they represent

## Labels
bug, ui