wallpaper: persist cycling state to disk

fixes #1010

core/internal/server/wallpaper/state.go

@@ -0,0 +1,45 @@
+package wallpaper
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/AvengeMedia/DankMaterialShell/core/internal/log"
+	"github.com/AvengeMedia/DankMaterialShell/core/internal/utils"
+)
+
+type persistentState struct {
+	LastFires map[string]time.Time `json:"lastFires"`
+}
+
+func stateFilePath() string {
+	return filepath.Join(utils.XDGCacheHome(), "dms", "wallpaper-schedule.json")
+}
+
+func loadLastFires() map[string]time.Time {
+	data, err := os.ReadFile(stateFilePath())
+	if err != nil {
+		return map[string]time.Time{}
+	}
+	var state persistentState
+	if err := json.Unmarshal(data, &state); err != nil || state.LastFires == nil {
+		return map[string]time.Time{}
+	}
+	return state.LastFires
+}
+
+func saveLastFires(fires map[string]time.Time) {
+	path := stateFilePath()
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return
+	}
+	data, err := json.Marshal(persistentState{LastFires: fires})
+	if err != nil {
+		return
+	}
+	if err := os.WriteFile(path, data, 0o644); err != nil {
+		log.Warnf("wallpaper: failed to persist schedule state: %v", err)
+	}
+}