[FIX] point_of_sale: BlockUI badly rendered at PoS "frontend"

Before this commit, the `BlockUI` component from
the web module was not rendered properly.

The reason for this is that after PR #83811 was merged,
the css from `web/static/src/core/ui/block_ui.scss`
was replaced with non-semantic Bootstrap css classes
(e.g. .d-flex, .justify-content-center). Those css
classes were inaccessible to PoS frontend, because
the `point_of_sale` module does not load Bootstrap
code (a design decision).

In this commit, we restore the proper rendering
of the `BlockUI` component both for the "frontend"
of the PoS App.

task-2924335

X-original-commit: 29cbeae90790ff503167b053ec6d9e221a82434e
Part-of: odoo/odoo#97730

addons/point_of_sale/static/src/css/pos.css

@@ -1243,6 +1243,22 @@ td {
     padding-top:15px;
 }
 
+/*  ********* The BlockUI Component (frontend)  ********* */
+.o_blockUI {
+    position: fixed;
+    top: 0;
+    left: 0;
+    z-index: 1100;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    flex-direction: column;
+    height: 100vh;
+    width: 100vw;
+    background: #000000;
+    color: white;
+    opacity: 0.6;
+}
 
 /*  ********* The Screens  ********* */