feat: add navigation and window size handlers with corresponding tests

internal/handlers/mocks_test.go

@@ -0,0 +1,31 @@
+package handlers_test
+
+import (
+	"errors"
+
+	"github.com/StevanFreeborn/term-tier/internal/core"
+)
+
+type mockSaver struct {
+	shouldError  bool
+	dataToReturn core.SaveState
+	savedData    core.SaveState
+}
+
+func (m *mockSaver) Save(filename string, state core.SaveState) error {
+	if m.shouldError {
+		return errors.New("mock save error")
+	}
+
+	m.savedData = state
+
+	return nil
+}
+
+func (m *mockSaver) Load(filename string) (core.SaveState, error) {
+	if m.shouldError {
+		return core.SaveState{}, errors.New("mock load error")
+	}
+
+	return m.dataToReturn, nil
+}