Use modern glow effect for dark theme + auto accent elements

Replace layered shadows with soft diffused glow:
- Centered blur (0 offset) for even light spread
- 16-24px blur radius for soft edges
- Light spread (1-4px) for subtle expansion

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

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

frontend/src/styles/Sidebar.css

@@ -1026,10 +1026,7 @@
 [data-theme='dark'][data-accent='auto'] .nav-item.active {
   background: #e5e7eb;
   color: #111827 !important;
-  box-shadow:
-    0 6px 8px rgba(229, 231, 235, 0.5),
-    0 12px 20px rgba(229, 231, 235, 0.25),
-    0 20px 35px rgba(229, 231, 235, 0.08);
+  box-shadow: 0 0 20px 2px rgba(229, 231, 235, 0.4);
 }
 
 [data-theme='dark'][data-accent='auto'] .nav-item.active .nav-icon,