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/TopBar.tsx

@@ -126,7 +126,7 @@ export function TopBar() {
             <button
               onClick={() => setUserMenuOpen(!userMenuOpen)}
               className="flex h-8 w-8 items-center justify-center rounded-[10px] text-xs font-heading font-bold text-white hover:opacity-90 transition-opacity"
-              style={{ background: 'linear-gradient(135deg, #ea580c, #f97316)' }}
+              style={{ background: 'linear-gradient(135deg, #3b82f6, #60a5fa)' }}
               title={user?.name || user?.email || 'User'}
             >
               {initials}