Change OAuth buttons to two-column layout (#5530)

Match Unity client layout: left column (Discord, GitHub, Steam),
right column (Google, Apple, Twitch). Includes responsive fallback
to single column on narrow screens.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/main/go/net/eagle0/admin_server/templates/login.html

@@ -98,11 +98,17 @@
 }
 
 .login-buttons {
-    display: flex;
-    flex-direction: column;
+    display: grid;
+    grid-template-columns: 1fr 1fr;
     gap: 1rem;
 }
 
+@media (max-width: 480px) {
+    .login-buttons {
+        grid-template-columns: 1fr;
+    }
+}
+
 .login-btn {
     display: flex;
     align-items: center;

src/main/go/net/eagle0/authservice/invitation_handlers.go

@@ -731,11 +731,16 @@ var landingPageTemplate = template.Must(template.New("landing").Parse(`<!DOCTYPE
             text-align: center;
         }
         .oauth-buttons {
-            display: flex;
-            flex-direction: column;
+            display: grid;
+            grid-template-columns: 1fr 1fr;
             gap: 12px;
             margin: 30px 0;
         }
+        @media (max-width: 480px) {
+            .oauth-buttons {
+                grid-template-columns: 1fr;
+            }
+        }
         .oauth-button {
             display: flex;
             align-items: center;