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/dashboard/PendingEscalations.tsx

@@ -3,16 +3,7 @@ import { Link, useNavigate } from 'react-router-dom'
 import { AlertTriangle } from 'lucide-react'
 import { aiSessionsApi } from '@/api/aiSessions'
 import type { AISessionSummary } from '@/types/ai-session'
-
-function timeAgo(dateStr: string): string {
-  const diffMs = Date.now() - new Date(dateStr).getTime()
-  const minutes = Math.floor(diffMs / 60000)
-  if (minutes < 1) return 'just now'
-  if (minutes < 60) return `${minutes}m ago`
-  const hours = Math.floor(minutes / 60)
-  if (hours < 24) return `${hours}h ago`
-  return `${Math.floor(hours / 24)}d ago`
-}
+import { timeAgo } from '@/lib/timeAgo'
 
 export function PendingEscalations() {
   const [escalations, setEscalations] = useState<AISessionSummary[]>([])