feat: Stage 1 scaffold — config, API client, placeholder TUI

Sets up the full project structure following the hactl/gokapi-tui
architecture: strict Elm-pattern Bubbletea TUI, YAML config at
~/.config/terdut-tui/config.yaml, REST API client with Bearer auth,
self-update via GitHub Releases, and a three-section tab placeholder
(Alerts / Schedule / Users) that verifies server connectivity on startup.

internal/tui/model.go

@@ -0,0 +1,89 @@
+package tui
+
+import (
+	"time"
+
+	"github.com/charmbracelet/bubbles/help"
+	tea "github.com/charmbracelet/bubbletea"
+	"github.com/yeniklas/terdut-tui/internal/api"
+)
+
+type section int
+
+const (
+	sectionAlerts section = iota
+	sectionSchedule
+	sectionUsers
+)
+
+type mode int
+
+const (
+	modeDashboard mode = iota
+	modeDetail
+	modeSchedule
+	modeUsers
+)
+
+// tea.Msg types
+
+type connectedMsg struct{ serverURL string }
+type connectErrMsg struct{ err error }
+type tickMsg time.Time
+type clearStatusMsg struct{}
+
+// Model holds all UI state.
+type Model struct {
+	client          *api.Client
+	serverURL       string
+	refreshInterval time.Duration
+
+	activeSection section
+	mode          mode
+	width         int
+	height        int
+
+	connected bool
+	err       error
+	statusMsg string
+
+	help help.Model
+	keys keyMap
+}
+
+func NewModel(client *api.Client, serverURL string, refreshInterval time.Duration) Model {
+	return Model{
+		client:          client,
+		serverURL:       serverURL,
+		refreshInterval: refreshInterval,
+		activeSection:   sectionAlerts,
+		mode:            modeDashboard,
+		help:            help.New(),
+		keys:            keys,
+	}
+}
+
+func (m Model) Init() tea.Cmd {
+	return connectCmd(m.client)
+}
+
+func connectCmd(client *api.Client) tea.Cmd {
+	return func() tea.Msg {
+		if err := client.HealthCheck(); err != nil {
+			return connectErrMsg{err}
+		}
+		return connectedMsg{}
+	}
+}
+
+func tickCmd(interval time.Duration) tea.Cmd {
+	return tea.Tick(interval, func(t time.Time) tea.Msg {
+		return tickMsg(t)
+	})
+}
+
+func clearStatusCmd() tea.Cmd {
+	return tea.Tick(3*time.Second, func(time.Time) tea.Msg {
+		return clearStatusMsg{}
+	})
+}