feat: add cordon/uncordon endpoints and node action toggles

Co-authored-by: aider (openrouter/thinkingmachines/inkling-small:free) <aider@aider.chat>

frontend/index.html

@@ -52,6 +52,8 @@
         <button id="btn-describe">Describe</button>
         <button id="btn-logs">Logs</button>
         <button id="btn-delete" class="danger">Delete</button>
+        <button id="btn-cordon" style="display:none;">Cordon</button>
+        <button id="btn-uncordon" style="display:none;">Uncordon</button>
       </div>
       <div style="margin-top:0.75rem;">
         <label>Exec (non-interactive):</label>