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/AdminPanel.css

@@ -2266,20 +2266,14 @@
 [data-theme='dark'][data-accent='auto'] .btn-primary-modern {
     background: #e5e7eb;
     color: #111827;
-    box-shadow:
-        0 6px 8px rgba(229, 231, 235, 0.45),
-        0 12px 20px rgba(229, 231, 235, 0.2),
-        0 20px 35px rgba(229, 231, 235, 0.06);
+    box-shadow: 0 0 16px 1px rgba(229, 231, 235, 0.35);
 }
 
 [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 8px 10px rgba(229, 231, 235, 0.55),
-        0 16px 25px rgba(229, 231, 235, 0.28),
-        0 24px 40px rgba(229, 231, 235, 0.1);
+    box-shadow: 0 0 24px 3px rgba(229, 231, 235, 0.5);
 }
 
 /* Icon action buttons (edit) hover state with auto accent in dark mode */