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/views/chat/chat.go

@@ -5,7 +5,6 @@ import (
 
 	"git.mlow.ca/mlow/lmcli/pkg/api"
 	"git.mlow.ca/mlow/lmcli/pkg/tui/model"
-	"git.mlow.ca/mlow/lmcli/pkg/tui/shared"
 	"github.com/charmbracelet/bubbles/cursor"
 	"github.com/charmbracelet/bubbles/spinner"
 	"github.com/charmbracelet/bubbles/textarea"
@@ -72,11 +71,10 @@ const (
 )
 
 type Model struct {
-	*shared.ViewState
-	shared.Sections
-
 	// App state
 	App		*model.AppModel
+	Height int
+	Width int
 
 	// Chat view state
 	state           state // current overall status of the view
@@ -106,14 +104,9 @@ type Model struct {
 	elapsed    time.Duration
 }
 
-func (m Model) Initialized() bool {
-	return m.ViewState.Initialized
-}
-
-func Chat(app *model.AppModel, shared shared.ViewState) shared.ViewModel {
+func Chat(app *model.AppModel) *Model {
 	m := Model{
 		App: app,
-		ViewState: &shared,
 
 		state:        idle,
 		persistence:  true,
@@ -166,12 +159,10 @@ func Chat(app *model.AppModel, shared shared.ViewState) shared.ViewModel {
 	m.input.FocusedStyle.CursorLine = lipgloss.NewStyle()
 	m.input.FocusedStyle.Base = inputFocusedStyle
 	m.input.BlurredStyle.Base = inputBlurredStyle
-
-	return m
+	return &m
 }
 
-func (m Model) Init() tea.Cmd {
-	m.ViewState.Initialized = true
+func (m *Model) Init() tea.Cmd {
 	return tea.Batch(
 		m.waitForResponseChunk(),
 	)