Enhance shadows for dark theme + auto accent elements

Use thicker, 3-layer shadows with progressive downward fade:
- Layer 1: 6-8px offset, strong opacity (45-55%)
- Layer 2: 12-16px offset, medium opacity (20-28%)
- Layer 3: 20-24px offset, fading opacity (6-10%)

🤖 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 +1026,10 @@
 [data-theme='dark'][data-accent='auto'] .nav-item.active {
   background: #e5e7eb;
   color: #111827 !important;
-  box-shadow: 0 4px 12px rgba(229, 231, 235, 0.35), 0 8px 24px rgba(229, 231, 235, 0.15);
+  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);
 }
 
 [data-theme='dark'][data-accent='auto'] .nav-item.active .nav-icon,