Fixes bugs and improve UI for chapter 3

[Rishav159]

This PR does the following:

• Fixes the color inconsistencies among diagrams and legends.
• Makes the nodes in the bfs, dfs and step costs diagram circular.
• Removes edge labels from depth limited search and iterative depth limited search diagrams.
• Switches the positions of standard bfs and bi-directional bfs to have a 'before-after' comparison
• Switches the positions of bfs shortest path and lowest cost path in the step costs diagram for the same reason
• Changes the node count label of bi-directional bfs diagram from x to x nodes
• Fixes the bug of node count going up-down if the diagram is restarted quickly.
• Adds a hover effect to Node-Expansion diagrams to indicate the node is clickable and also to easily compare the frontier nodes with frontier list on the right.

Improvement on #57

[redblobgames]

Hmm, the lighter color for breadth first search looks better in the small graph with large circles than in the large graph with small circles. I guess this is one of those cases where the same color can look a little different based on the surroundings. We can tweak it again later.