fix(editor): undo now covers texture painting, fix stale buffer bug

- Extend undo/redo to snapshot alpha maps and texture layers alongside
  heights — texture painting operations are now fully undoable
- Bracket paint mode with beginStroke/endStroke like sculpt mode
- Fix stale static char buffer in quest objective loop (showed wrong
  objective's description when editing multiple objectives)
- Zero-initialize all quest UI text buffers for null termination safety
- Fix unused variable warnings in terrain_editor.cpp

tools/editor/editor_app.cpp

@@ -400,12 +400,12 @@ void EditorApp::processEvents() {
                         }
                     } else {
                         painting_ = true;
-                        if (mode_ == EditorMode::Sculpt)
+                        if (mode_ == EditorMode::Sculpt || mode_ == EditorMode::Paint)
                             terrainEditor_.beginStroke();
                     }
                 } else if (event.type == SDL_MOUSEBUTTONUP) {
                     painting_ = false;
-                    if (mode_ == EditorMode::Sculpt)
+                    if (mode_ == EditorMode::Sculpt || mode_ == EditorMode::Paint)
                         terrainEditor_.endStroke();
                 }
             }