Task 17: Add KPI flip card interaction

Add click-to-flip interaction on LatestResults metric cards:
- CSS perspective-based 3D flip (400ms ease-in-out)
- Front face shows value/label/sub, back shows explanation text
- Single-card accordion: only one card flipped at a time
- Keyboard accessible: Enter/Space to flip, aria-label with state
- prefers-reduced-motion: instant visibility swap, no 3D animation
- Back face: accent-light background, 12px secondary text

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/tiles/LatestResultsTile.tsx

@@ -1,4 +1,4 @@
-import React from 'react'
+import React, { useState, useCallback } from 'react'
 import { Card, CardHeader } from '../Card'
 import { kpis } from '@/data/kpis'
 import type { KPI } from '@/types/pmr'
@@ -11,14 +11,31 @@ const colorMap: Record<KPI['colorVariant'], string> = {
 
 interface MetricCardProps {
   kpi: KPI
+  isFlipped: boolean
+  onFlip: (id: string) => void
 }
 
-function MetricCard({ kpi }: MetricCardProps) {
-  const cardStyles: React.CSSProperties = {
-    padding: '14px',
+function MetricCard({ kpi, isFlipped, onFlip }: MetricCardProps) {
+  const handleClick = () => {
+    onFlip(kpi.id)
+  }
+
+  const handleKeyDown = (e: React.KeyboardEvent) => {
+    if (e.key === 'Enter' || e.key === ' ') {
+      e.preventDefault()
+      onFlip(kpi.id)
+    }
+  }
+
+  const outerStyles: React.CSSProperties = {
     borderRadius: 'var(--radius-sm)',
     border: '1px solid var(--border-light)',
     background: 'var(--bg-dashboard)',
+    overflow: 'hidden',
+  }
+
+  const innerStyles: React.CSSProperties = {
+    padding: '14px',
   }
 
   const valueStyles: React.CSSProperties = {
@@ -43,16 +60,48 @@ function MetricCard({ kpi }: MetricCardProps) {
     marginTop: '4px',
   }
 
+  const backStyles: React.CSSProperties = {
+    padding: '14px',
+    background: 'var(--accent-light)',
+    fontSize: '12px',
+    color: 'var(--text-secondary)',
+    lineHeight: 1.5,
+    fontFamily: 'var(--font-ui)',
+    display: 'flex',
+    alignItems: 'center',
+  }
+
   return (
-    <div style={cardStyles} data-kpi-id={kpi.id}>
-      <div style={valueStyles}>{kpi.value}</div>
-      <div style={labelStyles}>{kpi.label}</div>
-      <div style={subStyles}>{kpi.sub}</div>
+    <div
+      className="metric-card"
+      style={outerStyles}
+      onClick={handleClick}
+      onKeyDown={handleKeyDown}
+      tabIndex={0}
+      role="button"
+      aria-label={`${kpi.label}: ${kpi.value}. ${isFlipped ? 'Showing explanation. Click to show value.' : 'Click to show explanation.'}`}
+    >
+      <div className={`metric-card-inner${isFlipped ? ' flipped' : ''}`}>
+        <div className="metric-card-front" style={innerStyles}>
+          <div style={valueStyles}>{kpi.value}</div>
+          <div style={labelStyles}>{kpi.label}</div>
+          <div style={subStyles}>{kpi.sub}</div>
+        </div>
+        <div className="metric-card-back" style={backStyles}>
+          {kpi.explanation}
+        </div>
+      </div>
     </div>
   )
 }
 
 export function LatestResultsTile() {
+  const [flippedCardId, setFlippedCardId] = useState<string | null>(null)
+
+  const handleFlip = useCallback((id: string) => {
+    setFlippedCardId((prev) => (prev === id ? null : id))
+  }, [])
+
   const gridStyles: React.CSSProperties = {
     display: 'grid',
     gridTemplateColumns: '1fr 1fr',
@@ -64,7 +113,12 @@ export function LatestResultsTile() {
       <CardHeader dotColor="teal" title="LATEST RESULTS" rightText="Updated May 2025" />
       <div style={gridStyles}>
         {kpis.map((kpi) => (
-          <MetricCard key={kpi.id} kpi={kpi} />
+          <MetricCard
+            key={kpi.id}
+            kpi={kpi}
+            isFlipped={flippedCardId === kpi.id}
+            onFlip={handleFlip}
+          />
         ))}
       </div>
     </Card>