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/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 {