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/sidebar/SidebarStatsBar.tsx

@@ -69,7 +69,7 @@ export function SidebarStatsBar({ resolved, active, completedMinutes, activeSess
       <div className="flex-1 rounded-md bg-[rgba(255,255,255,0.02)] px-1 py-1.5 text-center">
         <div
           className="font-mono text-sm font-semibold leading-none"
-          style={{ color: '#f97316' }}
+          style={{ color: '#60a5fa' }}
           aria-label={`${active} active sessions`}
         >
           {active}