feat: add Arch update helper and web UI

internal/state/state.go

@@ -0,0 +1,43 @@
+package state
+
+import (
+	"encoding/json"
+	"errors"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+type Store struct {
+	LastReminder    time.Time `json:"last_reminder"`
+	LastUpdateCount int       `json:"last_update_count"`
+}
+
+func Load(path string) (Store, error) {
+	data, err := os.ReadFile(path)
+	if errors.Is(err, os.ErrNotExist) {
+		return Store{}, nil
+	}
+	if err != nil {
+		return Store{}, err
+	}
+
+	var store Store
+	if err := json.Unmarshal(data, &store); err != nil {
+		return Store{}, err
+	}
+	return store, nil
+}
+
+func Save(path string, store Store) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return err
+	}
+
+	data, err := json.MarshalIndent(store, "", "  ")
+	if err != nil {
+		return err
+	}
+	data = append(data, '\n')
+	return os.WriteFile(path, data, 0o644)
+}