fix: add missing brand-950 to Tailwind config

Root cause: brand-950 was used in 13+ files but never defined in
tailwind.config.ts (brand colors only went 50-900). All dark:bg-brand-950
classes produced no CSS output, making dark backgrounds invisible.

Fix: add brand-950 mapped to --accent-900 CSS variable (same as 900,
the darkest shade available). This makes all existing brand-950 classes
work across the entire codebase.

Co-Authored-By: claude-flow <ruv@ruv.net>

apps/web/src/components/blueprints/FieldCard.tsx

@@ -83,7 +83,7 @@ export function FieldCard({ field, overrides, onChange }: FieldCardProps) {
     <div
       className={`border rounded-lg transition-all ${
         isActive
-          ? "border-brand-400/60 bg-brand-50/40 shadow-sm dark:border-brand-600/50 dark:bg-brand-950/60"
+          ? "border-brand-400/60 bg-brand-50/40 shadow-sm dark:border-brand-600/50 dark:bg-brand-950/50"
           : "border-gray-200 bg-white dark:border-gray-700/50 dark:bg-gray-800"
       }`}
     >