feat: replace hardcoded orange hex values with blue equivalents

BrandLogo gradient, EmptyStateIllustrations SVGs, categoryColors,
landing page, brand SVG assets, and all remaining files.
Warning #eab308 → #fbbf24 (amber). categoryColors deduped.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/tree-editor/FlowCanvasNode.tsx

@@ -23,7 +23,7 @@ const NODE_TYPE_CONFIG: Record<Exclude<NodeType, 'answer'>, {
     label: 'Action',
     borderClass: 'border-l-4 border-l-yellow-500',
     badgeClass: 'bg-yellow-500/20 text-yellow-400',
-    minimapColor: '#eab308',
+    minimapColor: '#fbbf24',
   },
   solution: {
     icon: CheckCircle,

frontend/src/components/tree-editor/GlowEdge.tsx

@@ -54,14 +54,14 @@ export function GlowEdgeDefs() {
 
         {/* Downstream: accent brand */}
         <linearGradient id="edge-gradient-downstream" x1="0%" y1="0%" x2="0%" y2="100%">
-          <stop offset="0%" stopColor="#ea580c" />
-          <stop offset="100%" stopColor="#f97316" />
+          <stop offset="0%" stopColor="#3b82f6" />
+          <stop offset="100%" stopColor="#60a5fa" />
         </linearGradient>
 
         {/* Upstream: amber */}
         <linearGradient id="edge-gradient-upstream" x1="0%" y1="100%" x2="0%" y2="0%">
           <stop offset="0%" stopColor="#f59e0b" />
-          <stop offset="100%" stopColor="#eab308" />
+          <stop offset="100%" stopColor="#fbbf24" />
         </linearGradient>
 
         {/* Glow filters */}