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/layout/Sidebar.tsx

@@ -450,7 +450,7 @@ export function Sidebar() {
           >
             {/* Drawer header */}
             <div className="px-3 mb-3">
-              <h3 className="text-[0.6875rem] font-mono uppercase tracking-[0.12em] text-[#eab308]">
+              <h3 className="text-[0.6875rem] font-mono uppercase tracking-[0.12em] text-[#fbbf24]">
                 {activeFlyoutGroup.label}
               </h3>
             </div>