fix(ui): smooth desktop sidebar sticky transitions (#2528)

* fix(ui): blend vibrant sidebar transitions

* fix(ui): fade sidebar content below header

* fix(ui): smooth vibrant sidebar sticky headers

* fix(ui): scope sticky fade to desktop

* fix(ui): respect sticky header preference

packages/ui/src/styles/design-system.css

@@ -60,7 +60,6 @@
     --sidebar-accent-foreground: oklch(0.25 0.02 40); /* Dark warm text */
     --sidebar-border: oklch(0.85 0.02 70); /* Warm border */
     --sidebar-ring: oklch(0.65 0.2 55); /* Focus ring */
-    --sidebar-stuck-bg: var(--sidebar); /* Desktop sidebar sticky header background (match sidebar) */
   }
 
   .dark {
@@ -97,7 +96,6 @@
     --sidebar-accent-foreground: oklch(0.85 0.02 90); /* #cdccc3 */
     --sidebar-border: oklch(0.31 0.01 35); /* #393836 */
     --sidebar-ring: oklch(0.77 0.17 85); /* #edb449 */
-    --sidebar-stuck-bg: var(--sidebar); /* Desktop sidebar sticky header background (match sidebar) */
   }
 
   * {
@@ -142,14 +140,6 @@
     background-color: transparent !important;
   }
 
-  /* Stuck sidebar zone headers must fully mask the rows scrolling under
-     them. Electron's transparent/vibrancy windows break backdrop-filter
-     (electron#20357), so use the opaque sidebar tone instead. */
-  :root.desktop-runtime[data-oc-vibrancy] .oc-zone-header-backing {
-    background: var(--sidebar-stuck-bg) !important;
-    background-color: var(--sidebar-stuck-bg) !important;
-  }
-
   .font-sans {
     font-family: var(--font-sans, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif) !important;
   }