Add a read-only Settings page for the loaded configuration (closes #402)

The page at /settings, behind the login and linked from the
navigation bar, lists every field of the configuration the server
started with: each by its environment variable name, with the
README's description and the value in effect. METRICS_PASSWORD and
SENTRY_DSN show only as set or not set; their values never reach the
template. The handlers now take the loaded Config from the dependency
graph.

Model: opus-5-5

templates/navbar.html

@@ -19,6 +19,7 @@
         <div class="hidden md:flex items-center gap-4">
             {{if .User}}
             <a href="/hooks" class="btn-text">Webhooks</a>
+            <a href="/settings" class="btn-text">Settings</a>
             <a href="/user/{{.User.Username}}" class="btn-text">
                 <svg class="w-5 h-5 mr-1" fill="currentColor" viewBox="0 0 16 16">
                     <path d="M11 6a3 3 0 1 1-6 0 3 3 0 0 1 6 0z"/>
@@ -43,6 +44,7 @@
         <div class="flex flex-col gap-2">
             {{if .User}}
             <a href="/hooks" class="btn-text w-full text-left">Webhooks</a>
+            <a href="/settings" class="btn-text w-full text-left">Settings</a>
             <a href="/user/{{.User.Username}}" class="btn-text w-full text-left">Profile</a>
             {{if .CSRFToken}}
             <form method="POST" action="/pages/logout">