style(workspace): drop the side panel's divider line

The context panel beside Sources, categories, Settings and collections
drew a 1px outline-variant border against the content surface, whose
background, shadow and rounded top-left corner already mark the edge;
the line clashed with that corner. The border stays 1px but transparent,
so the panel width, the persisted sidebar widths and the resize handle's
position do not move. The handle keeps its hover and drag highlight.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/workspace/shell/feature/src/lib/workspace-shell/components/workspace-shell-context-sidebar/workspace-shell-context-sidebar.component.scss

@@ -12,7 +12,11 @@
 }
 
 .context-panel {
-    border-right: 1px solid var(--mat-sys-outline-variant);
+    // No divider line: the panel and the content surface differ in colour,
+    // and the content's shadow and rounded corner mark the edge. The border
+    // stays transparent so the panel keeps its width and the resize handle
+    // its place.
+    border-right: 1px solid transparent;
     background: var(--app-rail-bg, var(--mat-sys-surface-container-low));
     display: flex;
     flex-direction: column;
@@ -28,8 +32,7 @@
     overflow: hidden;
     transition:
         width 180ms ease,
-        padding 180ms ease,
-        border-right-color 180ms ease;
+        padding 180ms ease;
 }
 
 .context-panel--collapsed {
@@ -40,7 +43,6 @@
     // visible empty strip beside the channels rail while "collapsed".
     padding-left: 0 !important;
     padding-right: 0 !important;
-    border-right-color: transparent;
 
     .resize-handle {
         display: none;
@@ -168,7 +170,6 @@
         width: 100% !important;
         min-width: 0 !important;
         padding: 12px 10px 10px 12px;
-        border-right-color: var(--mat-sys-outline-variant);
     }
 
     .context-panel :is(.resize-handle) {