fix: sign-out redirect and Google OAuth user creation

- signOut() cleared the cookie but never navigated, leaving the user
  stuck on the current page — redirect to /login on success.
- better-auth's core "image" field had no matching Drizzle column
  (schema uses avatarUrl), so every OAuth signup threw
  BetterAuthError: unable_to_create_user, silently failing before a
  session was ever created. Remap via user.fields.image.

apps/web/lib/auth/server.ts

@@ -100,6 +100,9 @@ export const auth = betterAuth({
   },
 
   user: {
+    fields: {
+      image: "avatarUrl",
+    },
     additionalFields: {
       role: {
         type: "string",