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/styles.go

@@ -0,0 +1,41 @@
+package tui
+
+import "github.com/charmbracelet/lipgloss"
+
+var (
+	colorPrimary  = lipgloss.Color("69")  // blue
+	colorMuted    = lipgloss.Color("240") // gray
+	colorFiring   = lipgloss.Color("196") // red
+	colorResolved = lipgloss.Color("70")  // green
+	colorAccent   = lipgloss.Color("214") // orange
+
+	styleHeader = lipgloss.NewStyle().
+			Bold(true).
+			Foreground(colorPrimary).
+			Padding(0, 1)
+
+	styleTabActive = lipgloss.NewStyle().
+			Bold(true).
+			Foreground(lipgloss.Color("0")).
+			Background(colorPrimary).
+			Padding(0, 2)
+
+	styleTabInactive = lipgloss.NewStyle().
+				Foreground(colorMuted).
+				Padding(0, 2)
+
+	styleFooter = lipgloss.NewStyle().
+			Foreground(colorMuted)
+
+	styleStatus = lipgloss.NewStyle().
+			Foreground(colorAccent).
+			Bold(true)
+
+	styleError = lipgloss.NewStyle().
+			Foreground(colorFiring).
+			Bold(true)
+
+	styleFiring   = lipgloss.NewStyle().Foreground(colorFiring).Bold(true)
+	styleResolved = lipgloss.NewStyle().Foreground(colorResolved)
+	styleMuted    = lipgloss.NewStyle().Foreground(colorMuted)
+)