feat(editor): quest editor with objectives, rewards, and quest chains

- New Quest mode (key 6) with full quest creation panel:
  - Title, description, required level
  - Quest giver / turn-in NPC ID linkage
  - Up to 4 objectives: Kill, Collect, Talk, Explore, Escort, Use Object
  - Rewards: XP and gold
  - Quest chain support via nextQuestId linking
- Quest list showing all created quests with level and objective count
- Save quests to JSON (included in Export Zone package)
- Foundation for campaign system: create quest chains across NPCs,
  link objectives to placed creatures, build storylines

tools/editor/editor_app.cpp

@@ -660,6 +660,12 @@ void EditorApp::exportZone(const std::string& outputDir) {
         npcSpawner_.saveToFile(npcPath);
     }
 
+    // Save quests
+    if (questEditor_.questCount() > 0) {
+        std::string questPath = base + "/quests.json";
+        questEditor_.saveToFile(questPath);
+    }
+
     // Save placed objects
     if (objectPlacer_.objectCount() > 0) {
         std::string objPath = base + "/objects.json";