feat: model selection for given provider (#57)

* feat: model selection for given provider

* tweak: adjust cfg validation func, remove duplicated logic, consolidate agent updating into agent.go

* tweak: make the model dialog scrollable, adjust padding slightly for modal"

* feat: add provider selection, add hints, simplify some logic, add horizontal scrolling support, additional scroll indicators"

* remove nav help

* update docs

* increase number of visible models, make horizontal scroll "wrap"

* add provider popularity rankings