refactor: dashboard design critique — eliminate redundancy, differentiate sections

- Remove GreetingStatStrip (duplicated PerformanceCards data)
- Strip left-border accent from stat cards (AI slop pattern)
- Redesign KnowledgeBaseCards: icon grid → compact row list with icon badges
- Redesign TeamSummary: distinct inline-row layout, no longer identical twin
- Differentiate hover: stat cards use subtle border-hover, sessions keep springy lift
- Add loading skeletons to PerformanceCards, KnowledgeBaseCards, TeamSummary
- Add error state to PerformanceCards
- Extract timeAgo() to shared lib/timeAgo.ts (replaced 4 duplicates)
- Fix Skeleton bg-brand-border (undefined CSS var) → border-default
- Fix double text-xs text-[0.5625rem] class conflicts across dashboard

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

frontend/src/components/ui/Skeleton.tsx

@@ -6,7 +6,7 @@ export function Skeleton({ className, ...props }: SkeletonProps) {
   return (
     <div
       className={cn(
-        'animate-pulse rounded-lg bg-brand-border',
+        'animate-pulse rounded-lg bg-[var(--color-border-default)]',
         className
       )}
       {...props}