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/view.go

@@ -0,0 +1,87 @@
+package tui
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/charmbracelet/lipgloss"
+)
+
+var sectionNames = []string{"Alerts", "Schedule", "Users"}
+
+func (m Model) View() string {
+	if m.width == 0 {
+		return ""
+	}
+
+	header := m.renderHeader()
+	tabs := m.renderTabs()
+	body := m.renderBody()
+	footer := m.renderFooter()
+
+	return lipgloss.JoinVertical(lipgloss.Left, header, tabs, body, footer)
+}
+
+func (m Model) renderHeader() string {
+	title := styleHeader.Render("terdut-tui")
+	right := ""
+	if m.serverURL != "" {
+		right = styleMuted.Render(m.serverURL)
+	}
+
+	gap := m.width - lipgloss.Width(title) - lipgloss.Width(right)
+	if gap < 0 {
+		gap = 0
+	}
+	return title + strings.Repeat(" ", gap) + right
+}
+
+func (m Model) renderTabs() string {
+	var tabs []string
+	for i, name := range sectionNames {
+		if section(i) == m.activeSection {
+			tabs = append(tabs, styleTabActive.Render(name))
+		} else {
+			tabs = append(tabs, styleTabInactive.Render(name))
+		}
+	}
+	line := strings.Repeat("─", m.width)
+	return strings.Join(tabs, "") + "\n" + styleMuted.Render(line)
+}
+
+func (m Model) renderBody() string {
+	bodyHeight := m.height - 5 // header + tabs + separator + footer + help
+	if bodyHeight < 1 {
+		bodyHeight = 1
+	}
+
+	var content string
+	if m.err != nil {
+		content = styleError.Render(fmt.Sprintf("Error: %v", m.err))
+	} else if !m.connected {
+		content = styleMuted.Render("Connecting…")
+	} else if m.statusMsg != "" {
+		content = styleStatus.Render(m.statusMsg)
+	} else {
+		switch m.activeSection {
+		case sectionAlerts:
+			content = styleMuted.Render("Alert dashboard — coming in Stage 2")
+		case sectionSchedule:
+			content = styleMuted.Render("On-call schedule — coming in Stage 4")
+		case sectionUsers:
+			content = styleMuted.Render("User management — coming in Stage 5")
+		}
+	}
+
+	lines := strings.Split(content, "\n")
+	padding := (bodyHeight - len(lines)) / 2
+	if padding < 0 {
+		padding = 0
+	}
+	return strings.Repeat("\n", padding) + content
+}
+
+func (m Model) renderFooter() string {
+	helpView := m.help.ShortHelpView(m.keys.ShortHelp())
+	return "\n" + styleFooter.Render(helpView)
+}