panel: place drill-down back links under the heading

The user form had Back to users at the bottom of the card; all drill-down
pages now use a shared back_link template with a structural test.

Co-Authored-By: Claude <Composer 2.5> <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/view/templates/delivery.html

@@ -18,7 +18,7 @@
   <span class="st st-{{.Level}}">{{.Row.Status}}</span>
 </p>
 
-<a class="back" href="{{.BackURL}}">&larr; Back to deliveries</a>
+{{template "back_link" (back .BackURL "Back to deliveries")}}
 
 {{/* The two columns: what was recorded on the left, in what order it happened
      on the right. They are a pair — the facts are only worth reading against

internal/web/view/templates/domain_delete.html

@@ -1,7 +1,7 @@
 {{define "content"}}
 <h1>Delete {{.Domain.Name}}</h1>
 
-<a class="back" href="/domains/{{.Domain.ID}}">&larr; Back to {{.Domain.Name}}</a>
+{{template "back_link" (back (printf "/domains/%d" .Domain.ID) (printf "Back to %s" .Domain.Name))}}
 
 <div class="card">
   <h2>Confirm deletion</h2>

internal/web/view/templates/domain_detail.html

@@ -1,7 +1,7 @@
 {{define "content"}}
 <h1>{{.Domain.Name}}</h1>
 
-<a class="back" href="/domains">&larr; All domains</a>
+{{template "back_link" (back "/domains" "All domains")}}
 
 {{if .Flash}}<div class="flash">{{.Flash}}</div>{{end}}
 {{if .RateLimitErr}}<div class="flash error">{{.RateLimitErr}}</div>{{end}}

internal/web/view/templates/layout.html

@@ -107,6 +107,14 @@
      so its definition replaces the empty one. */}}
 {{define "wide"}}{{end}}
 
+{{/* back_link — up-navigation on drill-down pages. Invoke with the "back"
+     function, e.g. {{template "back_link" (back "/users" "Back to users")}}.
+     Place it directly under the page <h1> and above the cards (see .back in
+     panel.css). TestDrillDownPagesPlaceBackLinkAboveContent guards this. */}}
+{{define "back_link"}}
+<a class="back" href="{{.Href}}">&larr; {{.Label}}</a>
+{{end}}
+
 {{/* Navigation icons. Inline SVG rather than an icon font or sprite file: they
      inherit the link's colour through currentColor, cost no extra request, and
      need no exemption from the panel's "default-src 'self'" policy. Each is

internal/web/view/templates/user_form.html

@@ -1,6 +1,8 @@
 {{define "content"}}
 <h1>{{if .IsEdit}}Edit user{{else}}Create user{{end}}</h1>
 
+{{template "back_link" (back "/users" "Back to users")}}
+
 <div class="card narrow">
   {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
   <form method="post" action="{{if .IsEdit}}/users/{{.UserID}}{{else}}/users/new{{end}}">
@@ -33,6 +35,5 @@
     <button type="submit" name="action" value="delete" class="danger">Delete user</button>
     {{end}}
   </form>
-  <p class="muted"><a href="/users">Back to users</a></p>
 </div>
 {{end}}