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/base.css

@@ -7,6 +7,7 @@
 :root {
     --primary-color: #4CAF50;
     --primary-hover: #5cb860;
+    --primary-contrast: #ffffff;
     --danger-color: #f44336;
     --warning-color: #ff9800;
     --info-color: #2196F3;

server/src/wled_controller/static/css/cards.css

@@ -399,7 +399,7 @@ body.cs-drag-active .card-drag-handle {
     padding: 1px 5px;
     border-radius: 3px;
     background: var(--primary-color);
-    color: #fff;
+    color: var(--primary-contrast);
     font-weight: 600;
     vertical-align: middle;
     margin-right: 2px;

server/src/wled_controller/static/css/components.css

@@ -7,7 +7,7 @@
 
 .badge.processing {
     background: var(--primary-color);
-    color: white;
+    color: var(--primary-contrast);
 }
 
 .badge.idle {
@@ -80,7 +80,7 @@
 
 .btn-primary {
     background: var(--primary-color);
-    color: white;
+    color: var(--primary-contrast);
 }
 
 .btn-danger {

server/src/wled_controller/static/css/dashboard.css

@@ -259,7 +259,7 @@
     font-size: 0.7rem;
     font-weight: 600;
     background: var(--primary-color);
-    color: #fff;
+    color: var(--primary-contrast);
     flex-shrink: 0;
 }
 

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 {

server/src/wled_controller/static/css/modal.css

@@ -832,7 +832,7 @@
 
 .gradient-stop-bidir-btn.active {
     background: var(--primary-color);
-    color: #fff;
+    color: var(--primary-contrast);
     border-color: var(--primary-color);
     opacity: 1;
 }

server/src/wled_controller/static/css/patterns.css

@@ -73,11 +73,11 @@
 
 .stream-card-link:hover {
     background: var(--primary-color);
-    color: #fff;
+    color: var(--primary-contrast);
 }
 
 .stream-card-link:hover .icon {
-    color: #fff;
+    color: var(--primary-contrast);
 }
 
 @keyframes cardHighlight {

server/src/wled_controller/static/css/streams.css

@@ -578,7 +578,7 @@
 
 .stream-tab-btn.active .stream-tab-count {
     background: var(--primary-color);
-    color: #fff;
+    color: var(--primary-contrast);
 }
 
 .cs-expand-collapse-group {