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;