fix: silence expected console noise from OIDC discovery and router hydration

- ServerConfigsLoader: skip validateAuthMetadata when getAuthMetadata()
  rejects (404 on /auth_issuer means server uses traditional SSO, not
  native Matrix OIDC/MAS - this is expected and should not log errors)
- Router: use HydrateFallback={() => null} instead of hydrateFallbackElement={null}
  so react-router v7 counts it as truthy and suppresses the spurious warning

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/components/ServerConfigsLoader.tsx

@@ -31,10 +31,14 @@ export function ServerConfigsLoader({ children }: ServerConfigsLoaderProps) {
       const authMetadata = promiseFulfilledResult(result[2]);
       let validatedAuthMetadata: ValidatedAuthMetadata | undefined;
 
-      try {
-        validatedAuthMetadata = validateAuthMetadata(authMetadata);
-      } catch (e) {
-        console.error(e);
+      // Only validate if the server returned metadata — a rejected promise means no native
+      // Matrix OIDC (MSC3861/MAS), which is normal for servers using traditional SSO.
+      if (authMetadata !== undefined) {
+        try {
+          validatedAuthMetadata = validateAuthMetadata(authMetadata);
+        } catch (e) {
+          console.error(e);
+        }
       }
 
       return {