Refactor TUI rendering handling and general cleanup

Improves render handling by moving the responsibility of laying out the
whole UI from each view and into the main `tui` model. Our `ViewModel`
interface has now diverged from bubbletea's `Model` and introduces
individual `Header`, `Content`, and `Footer` methods for rendering those
UI elements.

Also moved away from using value receivers on our Update and View
functions (as is common across Bubbletea) to pointer receivers, which
cleaned up some of the weirder aspects of the code (e.g. before we
essentially had no choice but to do our rendering in `Update` in order
to calculate and update the final height of the main content's
`viewport`).

pkg/tui/shared/shared.go

@@ -9,15 +9,12 @@ import (
 type ViewModel interface {
 	Init() tea.Cmd
 	Update(tea.Msg) (ViewModel, tea.Cmd)
-	View() string
-	Initialized() bool // Return whether this view is initialized
-}
 
-type ViewState struct {
-	Initialized bool
-	Width  int
-	Height int
-	Err    error
+	// View methods
+	Header(width int) string
+	// Render the view's main content into a container of the given dimensions
+	Content(width, height int) string
+	Footer(width int) string
 }
 
 type View int
@@ -29,22 +26,12 @@ const (
 	//StateHelp
 )
 
-// a convenience struct for holding rendered content for indiviudal UI
-// elements
-type Sections struct {
-	Header  string
-	Content string
-	Error   string
-	Input   string
-	Footer  string
-}
-
 type (
 	// send to change the current state
 	MsgViewChange View
 	// sent to a state when it is entered
 	MsgViewEnter struct{}
-	// sent when an error occurs
+	// sent when a recoverable error occurs (displayed to user)
 	MsgError error
 	// sent when the view has handled a key input
 	MsgKeyHandled tea.KeyMsg