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/hooks/useAuthFlows.ts

@@ -1,5 +1,5 @@
 import { createContext, useContext } from 'react';
-import { IAuthData, MatrixError } from 'matrix-js-sdk';
+import { IAuthData, MatrixError, ValidatedAuthMetadata } from 'matrix-js-sdk';
 import { ILoginFlowsResponse } from 'matrix-js-sdk/lib/@types/auth';
 
 export enum RegisterFlowStatus {
@@ -44,6 +44,13 @@ export const parseRegisterErrResp = (matrixError: MatrixError): RegisterFlowsRes
 export type AuthFlows = {
   loginFlows: ILoginFlowsResponse;
   registerFlows: RegisterFlowsResponse;
+  /**
+   * The homeserver's OAuth 2.0 metadata (`/_matrix/client/v1/auth_metadata`),
+   * when it uses native OAuth (MSC3861, e.g. matrix.org via MAS). The same
+   * source the SDK uses to refresh tokens, so sign-in is only offered when
+   * refresh will work too.
+   */
+  authMetadata?: ValidatedAuthMetadata;
 };
 
 const AuthFlowsContext = createContext<AuthFlows | null>(null);