feat: add navigation and window size handlers with corresponding tests

internal/handlers/downNavigationHandler.go

@@ -0,0 +1,41 @@
+package handlers
+
+import (
+	"slices"
+
+	"github.com/StevanFreeborn/term-tier/internal/state"
+	tea "github.com/charmbracelet/bubbletea"
+)
+
+type downNavigationHandler struct {
+	keys        []string
+	description string
+}
+
+func NewDownNavigationHandler() KeyMsgHandler {
+	return downNavigationHandler{
+		keys:        []string{"down", "j"},
+		description: "Move down",
+	}
+}
+
+func (h downNavigationHandler) KeysHandled() []string {
+	return h.keys
+}
+
+func (h downNavigationHandler) Description() string {
+	return h.description
+}
+
+func (h downNavigationHandler) Match(s state.State, msg tea.KeyMsg) bool {
+	if s.Mode() != state.NavigationMode {
+		return false
+	}
+
+	return slices.Contains(h.keys, msg.String())
+}
+
+func (h downNavigationHandler) Handle(s state.State, msg tea.KeyMsg) tea.Cmd {
+	s.MoveDown()
+	return nil
+}