Task 20: Accessibility audit improvements

Semantic HTML:
- Changed Card component from div to article element
- Added id="main-content" to main element for skip link target

Keyboard Navigation & ARIA:
- Added skip link to TopBar (visible only on focus, navigates to #main-content)
- Added aria-label="Active session information" to session info container
- Added aria-hidden="true" to all decorative colored dots (CardHeader, CareerActivity, Projects, Sidebar status badge)
- All expandable items already have role="button", tabIndex={0}, aria-expanded
- All KPI cards already have proper aria-label describing flip state
- Command palette already has full ARIA implementation (combobox, listbox, dialog)

Focus Management:
- Added global focus-visible styles in index.css (2px accent outline, 2px offset)
- Buttons, links, inputs all have proper focus rings with accent color
- Command palette focus trap already implemented

Reduced Motion:
- All components already check prefers-reduced-motion at module scope
- Dashboard entrance, tile expansion, KPI flip, palette animations respect reduced motion
- Added reduced motion override for pulse animation (disables pulse, keeps static dot)

Color Contrast:
- All color tokens already meet WCAG AA standards per ref spec
- Tertiary text (#8DA8A5) used only for supplementary labels where information is conveyed elsewhere

Quality checks: typecheck ✓, lint ✓ (1 pre-existing warning), build ✓

src/components/Card.tsx

@@ -23,7 +23,7 @@ export function Card({ children, full, className, tileId }: CardProps) {
   }
 
   return (
-    <div
+    <article
       style={baseStyles}
       className={className}
       data-tile-id={tileId}
@@ -31,7 +31,7 @@ export function Card({ children, full, className, tileId }: CardProps) {
       onMouseLeave={() => setIsHovered(false)}
     >
       {children}
-    </div>
+    </article>
   )
 }
 
@@ -85,7 +85,7 @@ export function CardHeader({ dotColor, title, rightText }: CardHeaderProps) {
 
   return (
     <div style={headerStyles}>
-      <div style={dotStyles} />
+      <div style={dotStyles} aria-hidden="true" />
       <span style={titleStyles}>{title}</span>
       {rightText && <span style={rightTextStyles}>{rightText}</span>}
     </div>