Splitting app into server_*.R files

[amcim]

This PR finishes the reorganization of the repo, splitting app.R into multiple server_*.R modules in line with utils/plots. A new script was created for

R/server_setup.R : Shared reactives + cross-tab UI-update observers
R/server_metadata.R : Metadata tab
R/server_modelperf.R : Model Performance + Performance overview
R/server_featureimportance.R : Bug/Drug feature comparison
R/server_crossmodel.R : Model holdouts
R/server_network.R : Drug-feature network
R/server_query.R : Query data + CSV downloads

setupServerCore(input, output, session, results_root) runs once inside the dashboard's server function and returns a named list of shared reactives

Each server_<tab>(input, output, session, core, ...) function then registers its outputs and observers, reading shared state via core$x() and forwarding results_root / amrdata_root

Testing

Run devtools::test() and load the dashboard with loadAMRDashboard() to make sure the plots still render correctly

[eboyer221]

Reviewed and tested locally, looks really good. Nice split of the server into
per-tab modules. Approving!