Enhance shadows for dark theme + auto accent elements

Use thicker, layered shadows with downward fade effect:
- Double shadow layers for depth
- Higher offset for bottom diffusion
- Increased opacity for better visibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/styles/Sidebar.css

@@ -1026,7 +1026,7 @@
 [data-theme='dark'][data-accent='auto'] .nav-item.active {
   background: #e5e7eb;
   color: #111827 !important;
-  box-shadow: var(--shadow-md);
+  box-shadow: 0 4px 12px rgba(229, 231, 235, 0.35), 0 8px 24px rgba(229, 231, 235, 0.15);
 }
 
 [data-theme='dark'][data-accent='auto'] .nav-item.active .nav-icon,