graph: highlight commits/dependencies on mouseover

When hovering over a commit, all dependent and depending commits should be highlighted, and also the corresponding arrows between them. Similarly when hovering over a line, all dependent and depending commits should be highlighted.

Highlighting could be achieved simply by increasing the border / line thickness.

Start by locating the graph visualization entry point and the handlers for commit and line mouseover. Trace how commits, dependency relationships, and arrows are represented, then verify that hovering either a commit or a line highlights the related commits and connecting arrows with increased border or line thickness.

Written by the indexing model from the issue text.