explore_model: view rhat, gradient, other diagnostic codes in ts panel

could look very similar to explore_timeseries, with three drop-downs for variables, etc. but where the options are the columns of get_fit (and possibly also predict_metab (for CIs) and predict_DO). all the ts stuff should be in one tab pane in explore_model

Start by comparing explore_model with explore_timeseries and identify how the existing time-series panel and tab panes are organized. Review the columns available from get_fit, plus the possible predict_metab and predict_DO outputs, and define the completed work as one explore_model tab showing the requested diagnostic and prediction variables through drop-downs.

Written by the indexing model from the issue text.