Fix shutdown banner visibility on Plugin Manager pages (#26234)

* Fix shutdown banner visibility on Plugin Manager pages

The shutdown banner was being obscured by the sticky app bar's
backdrop blur effect on the Installed, Available, and Updates pages.

Root cause: The sticky app bar (introduced in c42ab43) uses
pseudo-elements with backdrop-filter: blur(10px) that extend upwards
(negative top inset). These pseudo-elements have z-index: -1 relative
to the sticky bar (which has z-index: 2), but the shutdown banner had
no explicit z-index (defaulting to 0).

Fix: Add position: relative and z-index: 5 to #shutdown-msg and
#safe-restart-msg to ensure they render above the sticky app bar's
backdrop effect.

This is a minimal fix that preserves the header redesign while ensuring
critical system messages remain visible.

Fixes #26022

* Fix banner on dashboard experimental page

---------

Co-authored-by: Dhanush Raja A <dhanushrajaa@Dhanushs-MacBook-Air.local>
Co-authored-by: Tim Jacomb <timjacomb1@gmail.com>
Co-authored-by: Mark Waite <mark.earl.waite@gmail.com>

Author: 4 people

src/main/scss/base/_style.scss

@@ -71,6 +71,8 @@ td.no-wrap {
   padding: 0.5em;
   -moz-border-radius: 0.5em;
   border-radius: var(--form-input-border-radius);
+  position: relative;
+  z-index: 103;
 }
 
 #shutdown-msg {
@@ -82,6 +84,8 @@ td.no-wrap {
   padding: 0.5em;
   -moz-border-radius: 0.5em;
   border-radius: var(--form-input-border-radius);
+  position: relative;
+  z-index: 103;
 }
 
 a.lowkey:link {