US-004: Create SubNav component and useActiveSection hook

- Create SubNav component with sticky positioning below TopBar
- 5 sections: Overview, Skills, Experience, Projects, Education
- Active tab indicated with teal underline and 200ms slide transition
- Click scrolls smoothly to corresponding tile via data-tile-id
- Create useActiveSection hook using IntersectionObserver
- Maps tile IDs to section IDs for navigation
- Integrate SubNav into DashboardLayout with adjusted margins
- All styles follow design system (--accent, --surface, --border-light)
- TypeScript strict typing throughout

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

src/components/DashboardLayout.tsx

@@ -1,6 +1,7 @@
 import { useState, useEffect, useCallback } from 'react'
 import { motion } from 'framer-motion'
 import { TopBar } from './TopBar'
+import { SubNav } from './SubNav'
 import Sidebar from './Sidebar'
 import { CommandPalette } from './CommandPalette'
 import { PatientSummaryTile } from './tiles/PatientSummaryTile'
@@ -10,6 +11,7 @@ import { LastConsultationTile } from './tiles/LastConsultationTile'
 import { CareerActivityTile } from './tiles/CareerActivityTile'
 import { EducationTile } from './tiles/EducationTile'
 import { ProjectsTile } from './tiles/ProjectsTile'
+import { useActiveSection } from '@/hooks/useActiveSection'
 import type { PaletteAction } from '@/lib/search'
 
 const prefersReducedMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches
@@ -48,6 +50,7 @@ const contentVariants = {
 
 export function DashboardLayout() {
   const [commandPaletteOpen, setCommandPaletteOpen] = useState(false)
+  const activeSection = useActiveSection()
 
   const handleSearchClick = () => {
     setCommandPaletteOpen(true)
@@ -57,6 +60,11 @@ export function DashboardLayout() {
     setCommandPaletteOpen(false)
   }, [])
 
+  const handleSectionClick = useCallback((_sectionId: string) => {
+    // Section click is already handled in SubNav component
+    // This is just a placeholder for any additional logic needed
+  }, [])
+
   // Global Ctrl+K listener to open command palette
   useEffect(() => {
     function handleKeyDown(e: KeyboardEvent) {
@@ -114,12 +122,15 @@ export function DashboardLayout() {
         <TopBar onSearchClick={handleSearchClick} />
       </motion.div>
 
-      {/* Layout below TopBar: Sidebar + Main */}
+      {/* SubNav — sticky below TopBar */}
+      <SubNav activeSection={activeSection} onSectionClick={handleSectionClick} />
+
+      {/* Layout below TopBar + SubNav: Sidebar + Main */}
       <div
         style={{
           display: 'flex',
-          marginTop: 'var(--topbar-height)',
+          marginTop: 'calc(var(--topbar-height) + var(--subnav-height))',
-          height: 'calc(100vh - var(--topbar-height))',
+          height: 'calc(100vh - var(--topbar-height) - var(--subnav-height))',
         }}
       >
         {/* Sidebar — hidden on mobile/tablet, visible on desktop */}

src/components/SubNav.tsx

@@ -0,0 +1,88 @@
+interface NavSection {
+  id: string
+  label: string
+  tileId: string  // data-tile-id to scroll to
+}
+
+interface SubNavProps {
+  activeSection: string
+  onSectionClick: (sectionId: string) => void
+}
+
+const sections: NavSection[] = [
+  { id: 'overview', label: 'Overview', tileId: 'patient-summary' },
+  { id: 'skills', label: 'Skills', tileId: 'core-skills' },
+  { id: 'experience', label: 'Experience', tileId: 'career-activity' },
+  { id: 'projects', label: 'Projects', tileId: 'projects' },
+  { id: 'education', label: 'Education', tileId: 'education' },
+]
+
+export function SubNav({ activeSection, onSectionClick }: SubNavProps) {
+  const handleSectionClick = (section: NavSection) => {
+    // Scroll to the tile
+    const tileEl = document.querySelector(`[data-tile-id="${section.tileId}"]`)
+    if (tileEl) {
+      tileEl.scrollIntoView({ behavior: 'smooth', block: 'start' })
+    }
+    // Notify parent of section change
+    onSectionClick(section.id)
+  }
+
+  return (
+    <nav
+      aria-label="Section navigation"
+      style={{
+        position: 'sticky',
+        top: 'var(--topbar-height)',
+        zIndex: 99,
+        height: 'var(--subnav-height)',
+        background: 'var(--surface)',
+        borderBottom: '1px solid var(--border-light)',
+        display: 'flex',
+        alignItems: 'center',
+        justifyContent: 'center',
+        gap: '24px',
+      }}
+    >
+      {sections.map((section) => {
+        const isActive = activeSection === section.id
+
+        return (
+          <button
+            key={section.id}
+            onClick={() => handleSectionClick(section)}
+            aria-current={isActive ? 'true' : undefined}
+            style={{
+              position: 'relative',
+              fontSize: '13px',
+              fontWeight: 500,
+              color: isActive ? 'var(--accent)' : 'var(--text-secondary)',
+              background: 'none',
+              border: 'none',
+              padding: '0 0 2px 0',
+              cursor: 'pointer',
+              transition: 'color 200ms ease-out',
+              fontFamily: 'var(--font-ui)',
+            }}
+          >
+            {section.label}
+            {isActive && (
+              <span
+                style={{
+                  position: 'absolute',
+                  bottom: 0,
+                  left: 0,
+                  right: 0,
+                  height: '2px',
+                  background: 'var(--accent)',
+                  transition: 'all 200ms ease-out',
+                }}
+                aria-hidden="true"
+              />
+            )}
+          </button>
+        )
+      })}
+    </nav>
+  )
+}

src/hooks/useActiveSection.ts

@@ -0,0 +1,66 @@
+import { useState, useEffect } from 'react'
+
+// Map tile IDs to section IDs for SubNav
+const sectionTileMap: Record<string, string> = {
+  'patient-summary': 'overview',
+  'core-skills': 'skills',
+  'career-activity': 'experience',
+  'projects': 'projects',
+  'education': 'education',
+}
+
+/**
+ * Hook to track which section is currently visible using IntersectionObserver.
+ * Observes tiles by their data-tile-id attribute and maps them to section IDs.
+ *
+ * @returns The currently active section ID
+ */
+export function useActiveSection(): string {
+  const [activeSection, setActiveSection] = useState<string>('overview')
+
+  useEffect(() => {
+    // Find all tiles with data-tile-id attribute
+    const tiles = Array.from(
+      document.querySelectorAll('[data-tile-id]')
+    ) as HTMLElement[]
+
+    if (tiles.length === 0) return
+
+    // IntersectionObserver to track which tile is visible
+    const observer = new IntersectionObserver(
+      (entries) => {
+        // Find the entry with the highest intersection ratio
+        const visibleEntries = entries.filter((entry) => entry.isIntersecting)
+
+        if (visibleEntries.length === 0) return
+
+        // Get the most visible tile (highest intersection ratio)
+        const mostVisible = visibleEntries.reduce((prev, current) =>
+          current.intersectionRatio > prev.intersectionRatio ? current : prev
+        )
+
+        // Get the tile ID and map to section ID
+        const tileId = mostVisible.target.getAttribute('data-tile-id')
+        if (tileId && sectionTileMap[tileId]) {
+          setActiveSection(sectionTileMap[tileId])
+        }
+      },
+      {
+        // Trigger when tile is 25% visible
+        threshold: [0, 0.25, 0.5, 0.75, 1],
+        // Use viewport as root, with some margin for better UX
+        rootMargin: '-80px 0px -80% 0px',
+      }
+    )
+
+    // Observe all tiles
+    tiles.forEach((tile) => observer.observe(tile))
+
+    // Cleanup
+    return () => {
+      tiles.forEach((tile) => observer.unobserve(tile))
+    }
+  }, [])
+
+  return activeSection
+}