feat: add vercel configuration for webapp deployment

Co-authored-by: aider (anthropic/claude-sonnet-4-20250514) <[e-mail]>

package.json

···
     "doc": "docs"
   },
   "scripts": {
+    "build:webapp": "npm run build:types && npm run build --workspace=semble-webapp",
     "build:types": "npm run build --workspace=@semble/types",
     "dev:types": "npm run dev --workspace=@semble/types",
     "type-check": "tsc --noEmit",

vercel.json

···
+{
+  "buildCommand": "npm run build:webapp",
+  "outputDirectory": "src/webapp/.next",
+  "installCommand": "npm install"
+}