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/update.go

@@ -0,0 +1,54 @@
+package tui
+
+import (
+	tea "github.com/charmbracelet/bubbletea"
+)
+
+func (m Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
+	switch msg := msg.(type) {
+	case tea.WindowSizeMsg:
+		m.width = msg.Width
+		m.height = msg.Height
+		return m, nil
+
+	case connectedMsg:
+		m.connected = true
+		m.err = nil
+		m.statusMsg = "Connected to " + m.serverURL
+		return m, tea.Batch(tickCmd(m.refreshInterval), clearStatusCmd())
+
+	case connectErrMsg:
+		m.connected = false
+		m.err = msg.err
+		return m, nil
+
+	case tickMsg:
+		return m, tickCmd(m.refreshInterval)
+
+	case clearStatusMsg:
+		m.statusMsg = ""
+		return m, nil
+
+	case tea.KeyMsg:
+		return m.handleKey(msg)
+	}
+
+	return m, nil
+}
+
+func (m Model) handleKey(msg tea.KeyMsg) (Model, tea.Cmd) {
+	switch {
+	case msg.String() == "q" || msg.String() == "ctrl+c":
+		return m, tea.Quit
+
+	case msg.String() == "tab":
+		m.activeSection = (m.activeSection + 1) % 3
+		return m, nil
+
+	case msg.String() == "r":
+		m.statusMsg = "Refreshing…"
+		return m, tea.Batch(connectCmd(m.client), clearStatusCmd())
+	}
+
+	return m, nil
+}