feat: UI design system primitives, accessibility, and performance improvements

- Add Button component with CVA variants (primary, secondary, destructive, ghost, link)
- Add Input, Textarea, FormField, and Skeleton UI primitives
- Add focus trapping to Modal for WCAG accessibility compliance
- Add prefers-reduced-motion media query for motion-sensitive users
- Add route-level ErrorBoundary wrapping via page() helper in router
- Add route prefetching on sidebar nav hover for instant navigation
- Add PageMeta component with OG/Twitter meta tags (react-helmet-async)
- Add PageMeta to SharedSessionPage and SurveyPage for social sharing
- Replace lodash with custom debounce utility (saves ~71KB bundle)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -19,19 +19,18 @@
     "@dnd-kit/utilities": "^3.2.2",
     "@monaco-editor/react": "^4.7.0",
     "@stripe/stripe-js": "^8.7.0",
-    "@types/lodash": "^4.17.23",
     "@xyflow/react": "^12.10.0",
     "axios": "^1.13.4",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "date-fns": "^4.1.0",
     "immer": "^11.1.3",
-    "lodash": "^4.17.23",
     "lucide-react": "^0.563.0",
     "monaco-editor": "^0.55.1",
     "react": "^19.2.0",
     "react-day-picker": "^9.13.1",
     "react-dom": "^19.2.0",
+    "react-helmet-async": "^3.0.0",
     "react-markdown": "^10.1.0",
     "react-router-dom": "^7.13.0",
     "recharts": "^3.7.0",