[FIX] web: kanban muted quick create column

In grouped kanban, when having groups but no record to display, the
group creation button (ie. "+ Stage" in Project) and its related column
are muted instead of being the regular colors (like for the groups'
titles).

This commit fixes it by applying a fix similar to the group's title one
(cf. odoo/odoo@cbefc9cd51)
for the quick create column element.

Steps to reproduce:
- Open Project
- Create a new project and open it in kanban view
- create a few stages
=> the quick create column is "muted"
- dismiss the quick create column (ESC or click outside it)
=> the "+ Stage" button is muted

closes odoo/odoo#106022

X-original-commit: e95e2a23bc168815a27a9c64e4ff9394de468ada
Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>
Signed-off-by: Pierre Paridans (app) <app@odoo.com>

addons/web/static/src/legacy/scss/views.scss

@@ -46,7 +46,7 @@
 .o_view_nocontent {
     @include o-position-absolute(0, 0, 0, 0);
     pointer-events: none;
-    z-index: 1;
+    z-index: var(--o-view-nocontent-zindex, 1);
     display: flex;
     align-items: center;
     justify-content: center;

addons/web/static/src/views/kanban/kanban_controller.scss

@@ -22,6 +22,8 @@
 
     --KanbanColumn__highlight-background: #{rgba($o-info, 0.05)};
     --KanbanColumn__highlight-border: #{$o-info};
+
+    --o-view-nocontent-zindex: 1;
     // ----------------------------------------------------------------------------
 
     .dropdown,
@@ -500,9 +502,6 @@
         padding: 0 var(--KanbanGroup-padding-h) var(--KanbanGroup-padding-bottom);
 
         .o_kanban_header > .o_kanban_header_title {
-            // Makes the title come on top of the "no-content" background gradient.
-            z-index: 2;
-
             &:hover .o_kanban_config,
             .o_kanban_config.show {
                 visibility: visible;
@@ -526,6 +525,12 @@
         }
     }
 
+    .o_column_quick_create,
+    .o_kanban_group:not(.o_column_folded) .o_kanban_header_title {
+        // Makes them come on top of the "no-content" background gradient.
+        z-index: calc(var(--o-view-nocontent-zindex) + 1);
+    }
+
     .o_group_draggable .o_column_title {
         cursor: move;
     }