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/ActiveFlowPilotSessions.tsx

@@ -4,16 +4,7 @@ import { Clock, ArrowRight, Route, MessageCircle } from 'lucide-react'
 import { aiSessionsApi } from '@/api/aiSessions'
 import type { AISessionSummary } from '@/types/ai-session'
 import { cn } from '@/lib/utils'
-
-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 ActiveFlowPilotSessions({ hideHeader = false }: { hideHeader?: boolean }) {
   const [sessions, setSessions] = useState<AISessionSummary[]>([])
@@ -68,7 +59,7 @@ export function ActiveFlowPilotSessions({ hideHeader = false }: { hideHeader?: b
                 )}
                 <span
                   className={cn(
-                    'font-sans text-xs text-[0.5625rem] uppercase px-1.5 py-0.5 rounded',
+                    'font-sans text-[0.5625rem] uppercase px-1.5 py-0.5 rounded',
                     session.confidence_tier === 'guided' && 'bg-emerald-400/10 text-emerald-400',
                     session.confidence_tier === 'exploring' && 'bg-amber-400/10 text-amber-400',
                     session.confidence_tier === 'discovery' && 'bg-blue-400/10 text-blue-400',