Allow previewing an older NixOS release with --downgrade

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

pkgs/router-webui/templates/upgrade.html

@@ -18,8 +18,9 @@
     </label>
     <button class="secondary">Save</button>
   </form>
-  <p class="hint">The channel only chooses what an upgrade offers; saving it changes nothing on the router. NixOS cannot go back to
-     an older release.</p>
+  <p class="hint">The channel only chooses what an upgrade offers; saving it changes nothing on the router. Going back to an older release
+     (unstable to 26.05) is refused unless you tick "downgrade anyway" when previewing; state written by the newer
+     release may not work with the older one.</p>
 </section>
 
 {{if $d.Updating}}
@@ -54,6 +55,7 @@
   <form method="post" action="/upgrade/preview" class="actions">
     <input type="hidden" name="csrf" value="{{.CSRF}}">
     {{if and $d.Check $d.Check.UpToDate}}<label><input type="checkbox" name="force"> look anyway</label>{{end}}
+    <label><input type="checkbox" name="downgrade"> downgrade anyway</label>
     <button class="secondary"{{if or $d.Run.Running $d.Updating}} disabled{{end}}>Preview</button>
   </form>
   <p class="hint">Works it out from the new revision's description: no package is downloaded.</p>