refactor(styles): extract .app-scrollbar utility, apply to channel lists

Move the always-visible themed scrollbar from epg-list into a global
.app-scrollbar utility class in styles.scss. Apply it to the All
channels and Groups view virtual-scroll viewports so the channel list
gets the same persistent gutter and themed thumb as the EPG list.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: c470abf78379

libs/ui/components/src/lib/channel-list-container/all-channels-view/all-channels-view.component.html

@@ -12,7 +12,7 @@
     } @else {
         <cdk-virtual-scroll-viewport
             [itemSize]="itemSize()"
-            class="scroll-viewport"
+            class="scroll-viewport app-scrollbar"
         >
             <app-channel-list-item
                 *cdkVirtualFor="

libs/ui/components/src/lib/channel-list-container/groups-view/groups-view.component.html

@@ -206,7 +206,7 @@
 
                 <cdk-virtual-scroll-viewport
                     [itemSize]="itemSize()"
-                    class="groups-channels-viewport"
+                    class="groups-channels-viewport app-scrollbar"
                 >
                     <app-channel-list-item
                         *cdkVirtualFor="