Add SSO login support via mcdsl/sso

When [sso].redirect_uri is configured, the web UI shows a "Sign in
with MCIAS" button instead of the username/password form. Upgrades
mcdsl to v1.7.0 which includes the Firefox cookie fix.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/login.html

@@ -7,6 +7,11 @@
 </div>
 <div class="card">
     {{if .Error}}<div class="error">{{.Error}}</div>{{end}}
+    {{if .SSO}}
+    <div class="form-actions">
+        <a href="/sso/redirect" style="display:block;text-align:center;text-decoration:none;"><button type="button" style="width:100%" class="btn">Sign in with MCIAS</button></a>
+    </div>
+    {{else}}
     <form method="POST" action="/login">
         {{csrfField}}
         <div class="form-group">
@@ -25,5 +30,6 @@
             <button type="submit" class="btn">Login</button>
         </div>
     </form>
+    {{end}}
 </div>
 {{end}}