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/pages/account/BrandingSettingsPage.tsx

@@ -11,7 +11,7 @@ interface AccountBranding {
   company_name: string | null
 }
 
-const DEFAULT_COLOR = '#ea580c'
+const DEFAULT_COLOR = '#3b82f6'
 
 export function BrandingSettingsPage() {
   const [branding, setBranding] = useState<AccountBranding | null>(null)
@@ -207,7 +207,7 @@ export function BrandingSettingsPage() {
                       const val = e.target.value
                       setPrimaryColor(val)
                     }}
-                    placeholder="#ea580c"
+                    placeholder="#3b82f6"
                     maxLength={7}
                     className={cn(
                       'w-32 rounded-lg border border-border bg-card px-3 py-2 text-sm',
@@ -224,7 +224,7 @@ export function BrandingSettingsPage() {
                   </button>
                 </div>
                 <p className="mt-1 text-xs text-muted-foreground">
-                  Hex color code for the primary accent color (e.g. #ea580c).
+                  Hex color code for the primary accent color (e.g. #3b82f6).
                 </p>
               </div>
             </div>