ui(users): banner explaining the disabled-username re-enable flow

internal/server/http/ui_users.go

@@ -153,6 +153,11 @@ type userFormPage struct {
 	SetupURL   string
 	SetupExpAt time.Time
 	Error      string
+	// Reenable is set when the admin landed here because they tried
+	// to add a username that already exists (disabled). Triggers a
+	// banner on the edit page explaining why and steering them at
+	// the Re-enable button. See handleUIUserNewPost's collision branch.
+	Reenable bool
 }
 
 func (s *Server) handleUIUserNewGet(w stdhttp.ResponseWriter, r *stdhttp.Request) {
@@ -290,6 +295,7 @@ func (s *Server) handleUIUserEditGet(w stdhttp.ResponseWriter, r *stdhttp.Reques
 		Mode: "edit", ID: target.ID, Username: target.Username,
 		Email: em, Role: string(target.Role),
 		Disabled: target.DisabledAt != nil,
+		Reenable: r.URL.Query().Get("reenable") == "1",
 	}
 	_ = s.deps.UI.Render(w, "user_edit", view)
 }

web/templates/pages/user_edit.html

@@ -16,6 +16,28 @@
     {{else}}Edit <span class="mono">{{$page.Username}}</span>{{end}}
   </h1>
 
+  {{/* Re-enable banner — fires when the admin tried to add a user
+       whose name already exists in a disabled state. We were
+       redirected here from /settings/users/new with ?reenable=1. */}}
+  {{if and $page.Reenable $page.Disabled}}
+    <div class="panel mt-5 rounded-[7px] p-5"
+         style="border-color: color-mix(in oklch, var(--warn), transparent 50%);
+                background: color-mix(in oklch, var(--warn), transparent 95%);">
+      <div class="text-[13px] font-medium text-warn mb-2">Username already exists (disabled)</div>
+      <p class="text-pretty text-[12.5px] text-ink-mute leading-[1.6] mb-4">
+        A user with this name was created previously and then disabled.
+        Re-enable them to restore access (their existing role + email
+        are kept), or pick a different name.
+      </p>
+      <div class="flex gap-2">
+        <form method="post" action="/settings/users/{{$page.ID}}/enable">
+          <button type="submit" class="btn btn-primary">Re-enable {{$page.Username}}</button>
+        </form>
+        <a href="/settings/users/new" class="btn">Pick a different username</a>
+      </div>
+    </div>
+  {{end}}
+
   {{if eq $page.Mode "setup-link"}}
     {{if eq $page.Error "expired"}}
       <div class="panel mt-7 rounded-[7px] p-6"