Integrate MAS with Synapse

mas/share/templates/pages/upstream_oauth2/login_link.html

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+{#
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+-#}
+
+{% extends "base.html" %}
+
+{% block content %}
+  <header class="page-heading">
+    <div class="icon">
+      {{ icon.link() }}
+    </div>
+
+    <div class="header">
+      <h1 class="title">{{ _("mas.upstream_oauth2.login_link.heading") }}</h1>
+    </div>
+  </header>
+  <section class="flex flex-col gap-6 justify-center">
+    {{ _("mas.upstream_oauth2.login_link.description", username=linked_user.username) }}
+   
+    <form method="POST" class="cpd-form-root">
+      <input type="hidden" name="csrf" value="{{ csrf_token }}" />
+      <input type="hidden" name="action" value="link" />
+
+      {{ button.button(text=_("mas.upstream_oauth2.login_link.action")) }}
+    </form>
+
+  </section>
+{% endblock content %}