Render admin page errors in the normal layout (closes #382)

Every 400, 403, 404 and 500 on an admin page now answers with an
error page in the normal layout: one fixed line for the status and a
link back to the webhook list, or to sign-in when nobody is signed
in. The router's handler for unknown paths and the CSRF middleware's
refusal use the same page. Status codes are unchanged. The receiver,
the healthcheck and /metrics keep their plain answers. If the error
page itself cannot render, the answer is the same status in plain
text.

Model: opus-5-5

templates/navbar.html

@@ -24,10 +24,14 @@
                 </svg>
                 {{.User.Username}}
             </a>
+            {{/* The page for an unknown path is served outside the routes
+                 that issue a form token, and a logout without one is refused. */}}
+            {{if .CSRFToken}}
             <form method="POST" action="/pages/logout" class="inline">
                 <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
                 <button type="submit" class="btn-text">Logout</button>
             </form>
+            {{end}}
             {{else}}
             <a href="/pages/login" class="btn-primary">Login</a>
             {{end}}
@@ -40,10 +44,12 @@
             {{if .User}}
             <a href="/sources" class="btn-text w-full text-left">Webhooks</a>
             <a href="/user/{{.User.Username}}" class="btn-text w-full text-left">Profile</a>
+            {{if .CSRFToken}}
             <form method="POST" action="/pages/logout">
                 <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
                 <button type="submit" class="btn-text w-full text-left">Logout</button>
             </form>
+            {{end}}
             {{else}}
             <a href="/pages/login" class="btn-primary w-full">Login</a>
             {{end}}