feat(auth): offer native OAuth sign-in on servers like matrix.org

The login page looked for an OAuth issuer in .well-known
(m.authentication), which matrix.org no longer publishes, so its users
got the password / SSO compatibility forms. Detect native OAuth the way
the SDK finds the auth server for refresh: load /auth_metadata next to
the login flows. When present:
- Login shows "Continue with <server>" only (as Element does); a legacy
  SSO loginToken coming back from a redirect still completes.
- Register opens the auth server's sign-up page (prompt=create, where
  prompt_values_supported says so) instead of the legacy forms.
Servers without it (Lotus) are unchanged.

Checked against the live servers: matrix.org login and register show
"Continue with matrix.org" and no password form; matrix.lotusguild.org
keeps its password form.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/components/AuthFlowsLoader.tsx

@@ -23,7 +23,12 @@ export function AuthFlowsLoader({ fallback, error, children }: AuthFlowsLoaderPr
 
   const [state, load] = useAsyncCallback(
     useCallback(async () => {
-      const result = await Promise.allSettled([mx.loginFlows(), mx.registerRequest({})]);
+      const result = await Promise.allSettled([
+        mx.loginFlows(),
+        mx.registerRequest({}),
+        // Rejects when the server has no native OAuth: normal for most servers.
+        mx.getAuthMetadata(),
+      ]);
       const loginFlows = promiseFulfilledResult(result[0]);
       const registerResp = promiseRejectedResult(result[1]) as MatrixError | undefined;
       let registerFlows: RegisterFlowsResponse = { status: RegisterFlowStatus.InvalidRequest };
@@ -42,6 +47,7 @@ export function AuthFlowsLoader({ fallback, error, children }: AuthFlowsLoaderPr
       const authFlows: AuthFlows = {
         loginFlows,
         registerFlows,
+        authMetadata: promiseFulfilledResult(result[2]),
       };
 
       return authFlows;