feat(WEB-132): add seamless login flow

app/[lang]/(live)/(public)/login/route.ts

@@ -2,18 +2,59 @@ import { NextRequest, NextResponse } from "next/server"
 import { AuthError } from "next-auth"
 
 import { signIn } from "@/auth"
-import { badRequest } from "@/server/errors/next"
-
-import type { Lang } from "@/constants/languages"
+import { Lang } from "@/constants/languages"
+import { env } from "@/env/server"
+import { badRequest, internalServerError } from "@/server/errors/next"
 
 export async function GET(
   request: NextRequest,
   context: { params: { lang: Lang } }
 ) {
-  const redirectTo =
+  const returnUrl = request.headers.get("x-returnurl")
+
+  // If all else fails, always redirect to startpage
+  const finalDestination =
+    returnUrl ||
     request.headers.get("x-redirect-to") ||
     request.nextUrl.searchParams.get("redirectTo") ||
-    undefined
+    request.headers.get("Referer") ||
+    "/"
+
+  let redirectTo = finalDestination
+  if (!returnUrl) {
+    // This is a regular login request, not a seamless login request
+    // We should initiate the seamless login flow
+    try {
+      let redirectUrlValue
+      switch (context.params.lang) {
+        case Lang.da:
+          redirectUrlValue = env.SEAMLESS_LOGIN_DA
+          break
+        case Lang.de:
+          redirectUrlValue = env.SEAMLESS_LOGIN_DE
+          break
+        case Lang.en:
+          redirectUrlValue = env.SEAMLESS_LOGIN_EN
+          break
+        case Lang.fi:
+          redirectUrlValue = env.SEAMLESS_LOGIN_FI
+          break
+        case Lang.no:
+          redirectUrlValue = env.SEAMLESS_LOGIN_NO
+          break
+        case Lang.sv:
+          redirectUrlValue = env.SEAMLESS_LOGIN_SV
+          break
+      }
+      const redirectUrl = new URL(redirectUrlValue)
+      redirectUrl.searchParams.set("returnurl", finalDestination)
+      redirectTo = redirectUrl.toString()
+    } catch (e) {
+      console.error("Unable to create URL for seamless login")
+      console.error(e)
+      return internalServerError()
+    }
+  }
 
   try {
     /**