fix: add root vercel.json to enable API functions

The Vite framework preset doesn't automatically include API routes.
Adding vercel.json at repo root with explicit functions config.

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

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

Author: MaxGhenis

vercel.json

@@ -0,0 +1,15 @@
+{
+  "version": 2,
+  "buildCommand": "cd app && npm run build",
+  "outputDirectory": "app/dist",
+  "framework": null,
+  "functions": {
+    "app/api/*.ts": {
+      "runtime": "@vercel/node@3"
+    }
+  },
+  "rewrites": [
+    { "source": "/api/(.*)", "destination": "/app/api/$1" },
+    { "source": "/(.*)", "destination": "/app/dist/$1" }
+  ]
+}