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/layout/NotificationsPanel.tsx

@@ -10,14 +10,7 @@ import {
 } from 'lucide-react'
 import { notificationsApi } from '@/api/notifications'
 import type { AppNotification } from '@/types/notification'
-
-function timeAgo(dateStr: string): string {
-  const diff = Math.floor((Date.now() - new Date(dateStr).getTime()) / 1000)
-  if (diff < 60) return 'just now'
-  if (diff < 3600) return `${Math.floor(diff / 60)}m ago`
-  if (diff < 86400) return `${Math.floor(diff / 3600)}h ago`
-  return `${Math.floor(diff / 86400)}d ago`
-}
+import { timeAgo } from '@/lib/timeAgo'
 
 function EventIcon({ event }: { event: string }) {
   switch (event) {