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/DashboardLayout.tsx

@@ -135,6 +135,7 @@ export function DashboardLayout() {
 
         {/* Main content — scrollable card grid */}
         <motion.main
+          id="main-content"
           initial="hidden"
           animate="visible"
           variants={contentVariants}