fix: remove left borders from cards - use background color only

Per user request, removed border-left from success/info/warning cards.
Cards now distinguished by background color only, preserving border-radius.

Updated in:
- EmailRenderer.php: removed border-left from inline styles
- EditTemplate.tsx: removed border-left from CSS classes
- TemplateEditor.tsx: removed border-left from CSS classes

Card styling now:
- Success: #f0fdf4 (light green)
- Info: #f0f7ff (light blue)
- Warning: #fff8e1 (light yellow/cream)
- Hero: gradient background

admin-spa/src/routes/Settings/Notifications/EditTemplate.tsx

@@ -340,13 +340,13 @@ export default function EditTemplate() {
             .header { padding: 20px 16px; }
             .footer { padding: 20px 16px; }
           }
-          .card-success { background-color: #f0fdf4; border-left: 4px solid #22c55e; }
+          .card-success { background-color: #f0fdf4; }
           .card-highlight { background: linear-gradient(135deg, ${heroGradientStart} 0%, ${heroGradientEnd} 100%); color: ${heroTextColor}; }
           .card-highlight * { color: ${heroTextColor} !important; }
           .card-hero { background: linear-gradient(135deg, ${heroGradientStart} 0%, ${heroGradientEnd} 100%); color: ${heroTextColor}; }
           .card-hero * { color: ${heroTextColor} !important; }
-          .card-info { background-color: #f0f7ff; border-left: 4px solid #0071e3; }
-          .card-warning { background-color: #fff8e1; border-left: 4px solid #ff9800; }
+          .card-info { background-color: #f0f7ff; }
+          .card-warning { background-color: #fff8e1; }
           .card-basic { background: none; border: none; padding: 0; margin: 16px 0; }
           h1 { font-size: 26px; margin-top: 0; margin-bottom: 16px; color: #333; }
           h2 { font-size: 18px; margin-top: 0; margin-bottom: 16px; color: #333; }