fix: use flex-basis:auto to prevent container layout collapse

Replaces flex:1 1 0px with flex:1 1 auto in both the horizontal and vertical inner containers so TwinColGrid renders at its natural height when no explicit height is set.
Corrects the no-op min-width:0px to min-height:0px in the vertical container.

Close #201

Author: ngonzalezpazFC

src/main/java/com/flowingcode/vaadin/addons/twincolgrid/TwinColGrid.java

@@ -355,7 +355,7 @@ private HorizontalLayout createHorizontalContainer(boolean reverse) {
     }
     hl.setId(getLayoutId());
     hl.getElement().getStyle().set("min-height", "0px");
-    hl.getElement().getStyle().set("flex", "1 1 0px");
+    hl.getElement().getStyle().set("flex", "1 1 auto");
     hl.setMargin(false);
     hl.setWidthFull();
     return hl;
@@ -370,8 +370,8 @@ private VerticalLayout createVerticalContainer(boolean reverse) {
       vl = new VerticalLayout(available.layout, buttonContainer, selection.layout);
     }
     vl.setId(getLayoutId());
-    vl.getElement().getStyle().set("min-width", "0px");
-    vl.getElement().getStyle().set("flex", "1 1 0px");
+    vl.getElement().getStyle().set("min-height", "0px");
+    vl.getElement().getStyle().set("flex", "1 1 auto");
     vl.setMargin(false);
     vl.setPadding(false);
     vl.setHeightFull();