feat: add core types and styles for Tier management

internal/core/types.go

@@ -0,0 +1,23 @@
+package core
+
+import "github.com/charmbracelet/lipgloss"
+
+type Item string
+
+type Tier struct {
+	Name  string
+	Items []Item
+	Color lipgloss.Color
+}
+
+type SaveState struct {
+	Tiers []struct {
+		Name  string `json:"name"`
+		Items []Item `json:"items"`
+	} `json:"tiers"`
+}
+
+type Saver interface {
+	Save(filename string, state SaveState) error
+	Load(filename string) (SaveState, error)
+}