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-lock.json

@@ -14,19 +14,18 @@
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         "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",
@@ -2121,12 +2120,6 @@
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@@ -4643,6 +4636,15 @@
         "node": ">=12"
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@@ -4981,12 +4982,6 @@
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@@ -5004,6 +4999,18 @@
         "url": "https://github.com/sponsors/wooorm"
       }
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+    "node_modules/loose-envify": {
+      "version": "1.4.0",
+      "resolved": "https://registry.npmjs.org/loose-envify/-/loose-envify-1.4.0.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "js-tokens": "^3.0.0 || ^4.0.0"
+      },
+      "bin": {
+        "loose-envify": "cli.js"
+      }
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@@ -6335,6 +6342,26 @@
         "react": "^19.2.4"
       }
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+    "node_modules/react-fast-compare": {
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+      "resolved": "https://registry.npmjs.org/react-fast-compare/-/react-fast-compare-3.2.2.tgz",
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+      "license": "MIT"
+    },
+    "node_modules/react-helmet-async": {
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+      "resolved": "https://registry.npmjs.org/react-helmet-async/-/react-helmet-async-3.0.0.tgz",
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+      "license": "Apache-2.0",
+      "dependencies": {
+        "invariant": "^2.2.4",
+        "react-fast-compare": "^3.2.2",
+        "shallowequal": "^1.1.0"
+      },
+      "peerDependencies": {
+        "react": "^16.6.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/react-is": {
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       "resolved": "https://registry.npmjs.org/react-is/-/react-is-17.0.2.tgz",
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+    "node_modules/shallowequal": {
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+      "resolved": "https://registry.npmjs.org/shallowequal/-/shallowequal-1.1.0.tgz",
+      "integrity": "sha512-y0m1JoUZSlPAjXVtPPW70aZWfIL/dSP7AFkRnniLCrK/8MDKog3TySTBmckD+RObVxH0v4Tox67+F14PdED2oQ==",
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+    },
     "node_modules/shebang-command": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/shebang-command/-/shebang-command-2.0.0.tgz",