a11y(frontend): enhance accessibility support - Sprint 3 Story 5

Improve accessibility to meet WCAG 2.1 Level AA standards.

Changes: Added eslint-plugin-jsx-a11y, keyboard navigation, ARIA labels, SkipLink component, main-content landmark.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/(dashboard)/layout.tsx

@@ -4,6 +4,7 @@ import { Header } from '@/components/layout/Header';
 import { Sidebar } from '@/components/layout/Sidebar';
 import { useUIStore } from '@/stores/ui-store';
 import { AuthGuard } from '@/components/providers/AuthGuard';
+import { SkipLink } from '@/components/ui/skip-link';
 
 export default function DashboardLayout({
   children,
@@ -14,11 +15,13 @@ export default function DashboardLayout({
 
   return (
     <AuthGuard>
+      <SkipLink />
       <div className="min-h-screen">
         <Header />
         <div className="flex">
           <Sidebar />
           <main
+            id="main-content"
             className={`flex-1 transition-all duration-200 ${
               sidebarOpen ? 'ml-64' : 'ml-0'
             }`}