feat: add motd-assist TUI for managing dynamic MOTDs

Initial Go project built with Bubble Tea and Lip Gloss:
- Live preview and management of /etc/update-motd.d scripts
- Embedded presets (ubuntu-default, minimal, server-dashboard)
- Bundled POSIX sh style scripts (brand header, sysinfo panel, footer)
- Snapshot/restore safety around applying changes
- CLI subcommands preview, apply, restore, list-presets
- Unit tests for detect, sanitize, and apply logic

internal/ui/styles.go

@@ -0,0 +1,56 @@
+package ui
+
+import "github.com/charmbracelet/lipgloss"
+
+var (
+	colorAccent = lipgloss.Color("75")
+	colorBright = lipgloss.Color("81")
+	colorGood   = lipgloss.Color("114")
+	colorWarn   = lipgloss.Color("214")
+	colorBad    = lipgloss.Color("203")
+	colorDim    = lipgloss.Color("244")
+	colorBorder = lipgloss.Color("240")
+
+	focusedTitle = lipgloss.NewStyle().Bold(true).Foreground(colorBright)
+	dimStyle     = lipgloss.NewStyle().Foreground(colorDim)
+	goodStyle    = lipgloss.NewStyle().Foreground(colorGood)
+	badStyle     = lipgloss.NewStyle().Foreground(colorBad)
+	warnStyle    = lipgloss.NewStyle().Foreground(colorWarn)
+	cursorStyle  = lipgloss.NewStyle().Bold(true).Foreground(colorBright)
+	helpStyle    = lipgloss.NewStyle().Foreground(colorDim)
+)
+
+// paneBox baut den Rahmen-Stil für ein Pane mit gegebener Außenbreite/-höhe.
+// lipgloss zählt Padding zur Breite, den Rahmen nicht – daher w-2/h-2.
+func paneBox(focused bool, w, h int) lipgloss.Style {
+	s := lipgloss.NewStyle().
+		Border(lipgloss.RoundedBorder()).
+		BorderForeground(colorBorder).
+		PaddingLeft(1).
+		PaddingRight(1)
+	if focused {
+		s = s.BorderForeground(colorAccent)
+	}
+	if w > 0 {
+		s = s.Width(w - 2)
+	}
+	if h > 0 {
+		s = s.Height(h - 2)
+	}
+	return s
+}
+
+// truncate schneidet s Anzeigebreite auf w und hängt … an.
+func truncate(s string, w int) string {
+	if w <= 0 {
+		return ""
+	}
+	r := []rune(s)
+	if len(r) <= w {
+		return s
+	}
+	if w == 1 {
+		return "…"
+	}
+	return string(r[:w-1]) + "…"
+}