fix: increase z-index so toolboxes render above adjacent blocks

The .tc-wrap had z-index: 0 which created a stacking context that
trapped toolbox popovers behind adjacent Editor.js blocks. Increased
.tc-wrap z-index to 10 and .tc-toolbox z-index to 15 so the row/column
deletion popovers always appear in front of other content.

Fixes #186

Author: Codebuff Contributor

src/styles/table.pcss

@@ -19,7 +19,8 @@
   display: grid;
   grid-template-columns: calc(100% - var(--cell-size)) var(--cell-size);
   /* Bug-fix: https://github.com/editor-js/table/issues/175 */
-  z-index: 0;
+  /* Increased from 0 to 10 so toolboxes/popovers render above adjacent blocks */
+  z-index: 10;
 
   &--readonly {
     grid-template-columns: 100% var(--cell-size);

src/styles/toolboxes.pcss

@@ -7,7 +7,7 @@
 
   position: absolute;
   cursor: pointer;
-  z-index: 1;
+  z-index: 15;
   opacity: 0;
   transition: opacity 0.1s;