Remove plotly: Create copy of base/common functionality using matplotlib -- base_plot, config, util, constants

[georgemccabe]

[georgemccabe]

Met on 2026/01/21. Both working in branch feature_555_replace_plotly, we will copy the common/base functionality to create a matplotlib-based version so we can switch each plot type we work on to use the new common code. Eventually we will replace the plotly-based code with the matplotlib code so we don't have 2 versions.

@bikegeek will start with
plots/base_plot.py -> plots/base_plot_mpl.py

@georgemccabe will start with

plots/util.py -> plots/util_mpl.py

Coordinate on who will work on adding matplotlib versions of plotly-specific content in:
plots/constants.py
plots/config.py

@georgemccabe will start generating a list of commands to run to create test plots to create a baseline that we can use to compare to the updated versions.