Auto-compute contrast text color for accent backgrounds

Add --primary-contrast CSS variable that auto-switches between white and
dark text based on accent color luminance (WCAG relative luminance).
Replace all hardcoded #fff/white on primary-color backgrounds with
var(--primary-contrast) so light accent colors like yellow remain readable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/css/layout.css

@@ -161,7 +161,7 @@ h2 {
 
 .tab-btn.active .tab-badge {
     background: var(--primary-color);
-    color: #fff;
+    color: var(--primary-contrast);
 }
 
 .tab-panel {
@@ -410,11 +410,12 @@ h2 {
 
 .cp-result.cp-active {
     background: var(--primary-color);
-    color: #fff;
+    color: var(--primary-contrast);
 }
 
 .cp-result.cp-active .cp-detail {
-    color: rgba(255, 255, 255, 0.7);
+    color: var(--primary-contrast);
+    opacity: 0.7;
 }
 
 .cp-icon {