Make registration errors informative

- Backend: explicitly return 409 EMAIL_TAKEN when the email already
  exists, instead of Better Auth's silent 200 (common after migration).
- Frontend: split error handling — network failure, non-JSON body
  (502/proxy), and meaningful server errors get distinct messages; the
  "email already registered" case shows login / reset-password links
  instead of a generic "connection error".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/api/register/route.ts

@@ -1,5 +1,6 @@
 import { NextRequest } from "next/server";
 import { auth } from "@/lib/auth";
+import { prisma } from "@/lib/prisma";
 
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 const disposableDomains = require("disposable-email-domains") as string[];
@@ -21,8 +22,8 @@ async function verifyTurnstile(token: string): Promise<boolean> {
   }
 }
 
-function jsonError(message: string, status: number) {
-  return new Response(JSON.stringify({ message }), {
+function jsonError(message: string, status: number, code?: string) {
+  return new Response(JSON.stringify({ message, code }), {
     status,
     headers: { "Content-Type": "application/json" },
   });
@@ -59,6 +60,18 @@ export async function POST(request: NextRequest) {
     }
   }
 
+  // Email уже зарегистрирован? Сообщаем явно, а не молчаливым «успехом»
+  // от Better Auth — частый случай после миграции (у людей уже есть аккаунт).
+  if (email) {
+    const existing = await prisma.user.findFirst({
+      where: { email: { equals: email, mode: "insensitive" } },
+      select: { id: true },
+    });
+    if (existing) {
+      return jsonError("Аккаунт с таким email уже зарегистрирован", 409, "EMAIL_TAKEN");
+    }
+  }
+
   const baseUrl =
     process.env.BETTER_AUTH_URL ??
     `https://${request.headers.get("host")}`;