fix(seo): add trailingSlash to vercel.json for edge 308 redirects (#239)

The Next.js trailingSlash config only affects build output (generating
en/index.html vs en.html). Vercel static hosting still serves both /en
and /en/ as 200 without redirecting. Adding trailingSlash: true to
vercel.json makes Vercel enforce 308 /en → /en/ at the edge, so Semrush
sees a single canonical URL form.

Also updates root redirect / → /en/ to avoid double redirect.

Co-authored-by: paperclip-resolver[bot] <3736210+paperclip-resolver[bot]@users.noreply.github.com>

Author: Mlaz-code

vercel.json

@@ -7,7 +7,7 @@
   "redirects": [
     {
       "source": "/",
-      "destination": "/en",
+      "destination": "/en/",
       "permanent": true
     },
     {
@@ -189,5 +189,6 @@
       "source": "/ingest/:path*",
       "destination": "https://us.i.posthog.com/:path*"
     }
-  ]
+  ],
+  "trailingSlash": true
 }