Add unqueue (delete) button to web reading view

Adds a delete route and handler to the web UI so documents can be
removed directly from the reading page. Uses CSRF-protected POST with a
browser confirmation dialog. Styled with a danger accent.

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

web/templates/read.html

@@ -18,6 +18,11 @@
             <button type="submit" class="btn-ghost btn btn-sm">Mark read</button>
         </form>
         {{end}}
+        <form method="POST" action="/d/{{.Doc.Slug}}/delete" style="display:inline"
+              onsubmit="return confirm('Delete this document?')">
+            {{csrfField}}
+            <button type="submit" class="btn-ghost btn btn-sm btn-danger">Unqueue</button>
+        </form>
         <a href="/" class="btn-ghost btn btn-sm">Back to queue</a>
     </div>
 </div>