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/AdminPanel.css

@@ -2266,12 +2266,14 @@
 [data-theme='dark'][data-accent='auto'] .btn-primary-modern {
     background: #e5e7eb;
     color: #111827;
+    box-shadow: 0 4px 12px rgba(229, 231, 235, 0.3), 0 8px 20px rgba(229, 231, 235, 0.12);
 }
 
 [data-theme='dark'][data-accent='auto'] .btn-primary:hover,
 [data-theme='dark'][data-accent='auto'] .btn-primary-modern:hover {
     background: #f3f4f6;
     color: #111827;
+    box-shadow: 0 6px 16px rgba(229, 231, 235, 0.4), 0 12px 28px rgba(229, 231, 235, 0.18);
 }
 
 /* Icon action buttons (edit) hover state with auto accent in dark mode */