feat: Euler Path & Circuit Finder — interactive tool with Hierholzer's algorithm animation

Add interactive Euler path/circuit finder with:
- Draw/edit graphs with click-to-add nodes and edges
- Directed/undirected graph support
- Real-time degree analysis with odd-degree highlighting
- Hierholzer's algorithm with step-by-step animation
- 6 presets: Königsberg Bridges, Triangle, Rectangle+Diagonal, House, Petersen, K5
- Degree table, verdict panel, path display
- Speed control, step mode, right-click delete
- Multi-edge support with curved rendering

Usage: Open docs/euler.html or navigate from sidebar

Author: sauravbhattacharya001