refactor: implement icon rail sidebar for Design System v4

72px icon rail with hover flyouts, pin-to-expand toggle (260px),
keyboard accessible, mobile hamburger overlay. Flat TopBar styling
(no blur/glass). New BrandLogo mark (gradient square + lightning bolt).
BrandWordmark uses solid text color instead of gradient.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/common/BrandWordmark.tsx

@@ -9,13 +9,12 @@ export function BrandWordmark({ size = 'sm', className }: BrandWordmarkProps) {
   return (
     <span
       className={cn(
-        'font-heading font-bold tracking-tight',
+        'font-heading font-bold tracking-tight text-[#f0f2f5]',
         size === 'sm' ? 'text-xl' : 'text-3xl',
         className
       )}
     >
-      <span className="text-foreground">Resolution</span>
-      <span className="text-gradient-brand">Flow</span>
+      ResolutionFlow
     </span>
   )
 }