style: reset background color of paginators

Resolves #6799.

Removes the two `.mat-mdc-paginator` CSS overrides in
apps/client/src/styles.scss:

- The dark-theme block setting the background to
  `rgba(var(--palette-foreground-base-dark), 0.02)`
- The global block setting the background to
  `rgba(var(--palette-foreground-base-light), 0.02)` plus the
  `.mat-mdc-paginator-page-size { display: none }` rule (the
  selector was duplicated in the source)

The default Angular Material paginator background is transparent /
follows the surface theme, which matches the issue's request.
Components that need to hide the page-size selector should use the
paginator's `[hidePageSize]` API rather than a CSS override.

apps/client/src/styles.scss

@@ -258,10 +258,6 @@ body {
       }
     }
 
-    .mat-mdc-paginator {
-      background-color: rgba(var(--palette-foreground-base-dark), 0.02);
-    }
-
     .mdc-button {
       &.mat-accent,
       &.mat-primary {
@@ -459,15 +455,6 @@ ngx-skeleton-loader {
   }
 }
 
-.mat-mdc-paginator {
-  background-color: rgba(var(--palette-foreground-base-light), 0.02);
-
-  .mat-mdc-paginator-page-size,
-  .mat-mdc-paginator-page-size {
-    display: none;
-  }
-}
-
 .mat-stepper-vertical,
 .mat-stepper-horizontal {
   background: transparent !important;