US-003: Create DetailPanelContext, DetailPanel component, and useFocusTrap hook

Implements core detail panel infrastructure for slide-in content panels:

- DetailPanelContext: Manages panel state (content, open/close, isOpen)
- DetailPanel: Slide-in panel component with backdrop, header, and scrollable body
- useFocusTrap: Keyboard focus trap hook for modal accessibility
- Width mapping: narrow (400px) for kpi/skill/education, wide (60vw) for consultation/project/career-role
- Title mapping derives from content data (kpi.label, skill.name, etc.)
- Close triggers: backdrop click, Escape key, X button
- ARIA: aria-modal, role=dialog, aria-labelledby
- Mobile responsive: both widths become 100vw on <768px
- prefers-reduced-motion: instant appear, no animations
- Placeholder content (real renderers in later stories)
- Export CardHeaderProps interface from Card.tsx
- Add responsive panel width CSS rules

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/contexts/DetailPanelContext.tsx

@@ -0,0 +1,52 @@
+import { createContext, useContext, useState, ReactNode } from 'react'
+import { DetailPanelContent } from '@/types/pmr'
+
+interface DetailPanelContextValue {
+  content: DetailPanelContent | null
+  openPanel: (content: DetailPanelContent) => void
+  closePanel: () => void
+  isOpen: boolean
+}
+
+const DetailPanelContext = createContext<DetailPanelContextValue | undefined>(
+  undefined
+)
+
+interface DetailPanelProviderProps {
+  children: ReactNode
+}
+
+export function DetailPanelProvider({ children }: DetailPanelProviderProps) {
+  const [content, setContent] = useState<DetailPanelContent | null>(null)
+
+  const openPanel = (newContent: DetailPanelContent) => {
+    setContent(newContent)
+  }
+
+  const closePanel = () => {
+    setContent(null)
+  }
+
+  const isOpen = content !== null
+
+  const value: DetailPanelContextValue = {
+    content,
+    openPanel,
+    closePanel,
+    isOpen,
+  }
+
+  return (
+    <DetailPanelContext.Provider value={value}>
+      {children}
+    </DetailPanelContext.Provider>
+  )
+}
+
+export function useDetailPanel(): DetailPanelContextValue {
+  const context = useContext(DetailPanelContext)
+  if (!context) {
+    throw new Error('useDetailPanel must be used within DetailPanelProvider')
+  }
+  return context
+}