fix: Round 54 priority dequeue, onComplete callback, prerequisiteObjectiveId validation, localizationTable guard, FailQuest timestamp, remove empty ValidateBranchDialogueKeys

- DialogueManager.EndDialogue: dequeue by max-priority index instead of FIFO index-0
- DialogueManager.EndDialogue: fire _onCompleteCallback on normal end (was only in ForceEnd)
- NpcSO.OnValidate: auto-restore localizationTable to 'UI' if cleared
- QuestSO.ValidateObjectiveIds: validate prerequisiteObjectiveId references exist in same quest
- QuestSO.OnValidate: remove call to empty ValidateBranchDialogueKeys stub + remove the stub itself
- QuestManager.DispatchEvent toFail loop: write _completedAtUtc timestamp on quest failure

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Assets/_Game/Scripts/Dialogue/NpcSO.cs

@@ -68,6 +68,9 @@ namespace BaseGames.Dialogue
 
         private void OnValidate()
         {
+            if (string.IsNullOrEmpty(localizationTable))
+                localizationTable = "UI";
+
             if (string.IsNullOrWhiteSpace(npcId))
             {
                 UnityEditor.EditorUtility.SetDirty(this);