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/dashboard/StartSessionInput.tsx

@@ -346,7 +346,7 @@ export function StartSessionInput() {
             onClick={() => handleSuggestionClick(label)}
             className="group flex items-center gap-1.5 rounded-md border border-border bg-card px-3 py-1.5 text-xs text-muted-foreground transition-all hover:border-[var(--color-border-hover)] hover:bg-[var(--color-bg-elevated)] hover:text-foreground active:scale-[0.97]"
           >
-            <Icon size={11} className="text-muted shrink-0 group-hover:text-[#f97316] transition-colors" />
+            <Icon size={11} className="text-muted shrink-0 group-hover:text-[#60a5fa] transition-colors" />
             {label}
           </button>
         ))}