build(mui): add MUI v7 + Emotion + X pickers; align @types/react to 18

Install @mui/material@^7, @emotion/react@^11, @emotion/styled@^11,
@mui/x-date-pickers@^8 as production dependencies. Downgrade
@types/react and @types/react-dom from ^19 to ^18.3 to match the
React 18.3.1 runtime. tsc --noEmit and vite build:client both pass
cleanly with no new errors.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package.json

@@ -32,11 +32,15 @@
     "@dnd-kit/modifiers": "^9.0.0",
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
+    "@emotion/react": "^11.14.0",
+    "@emotion/styled": "^11.14.1",
     "@fastify/cookie": "^11.0.2",
     "@fastify/cors": "^11.2.0",
     "@fastify/multipart": "^9.4.0",
     "@fastify/rate-limit": "^10.3.0",
     "@fastify/static": "^9.0.0",
+    "@mui/material": "^7.3.11",
+    "@mui/x-date-pickers": "^8.29.0",
     "@prisma/client": "^6.19.2",
     "@tanstack/react-query": "^5.100.5",
     "@types/jsdom": "^28.0.1",
@@ -74,8 +78,8 @@
     "@types/node-cron": "^3.0.11",
     "@types/nodemailer": "^7.0.11",
     "@types/qrcode": "^1.5.6",
-    "@types/react": "^19.2.14",
-    "@types/react-dom": "^19.2.3",
+    "@types/react": "^18.3.31",
+    "@types/react-dom": "^18.3.7",
     "@types/supertest": "^7.2.0",
     "@vitejs/plugin-react": "^6.0.1",
     "concurrently": "^9.2.1",