fix(base): pull AddRowButton out of grid so sticky-left actually sticks

As a child of .grid with grid-column: 1/-1, the button took the full
row width and the sticky-left offset got swallowed by the grid layout
— it scrolled along with the cells instead of anchoring to the left.

Move it to a sibling of .grid inside .gridWrapper, restyle as a
small inline-flex chip with width: max-content. Sticky-left now
keeps it pinned to page-content-left while the user scrolls
horizontally, matching the toolbar.

Add padding-bottom: 6px on .gridWrapper and a small vertical margin
on the button so the horizontal scrollbar no longer overlaps it.

apps/client/src/features/base/components/grid/grid-container.tsx

@@ -296,10 +296,9 @@ export function GridContainer({
           {paddingBottom > 0 && (
             <div style={{ height: paddingBottom, gridColumn: "1 / -1" }} />
           )}
-
-          <AddRowButton onClick={handleAddRow} />
-          {pageId && <SelectionActionBar pageId={pageId} />}
         </div>
+        <AddRowButton onClick={handleAddRow} />
+        {pageId && <SelectionActionBar pageId={pageId} />}
       </div>
     </DndContext>
   );