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_ui.hpp

@@ -31,6 +31,7 @@ private:
     void renderObjectPanel(EditorApp& app);
     void renderWaterPanel(EditorApp& app);
     void renderNpcPanel(EditorApp& app);
+    void renderQuestPanel(EditorApp& app);
     void renderContextMenu(EditorApp& app);
     void renderMinimap(EditorApp& app);
     void renderPropertiesPanel(EditorApp& app);