Require Gitea login to write GPX data; IPv6-capable bind

Upload/delete/move/create-folder now require logging in via Gitea
OAuth2 (only MAPTOOL_ALLOWED_GITEA_USER is granted write access);
reading stays public. Deploy secrets (session secret, OAuth2 client
credentials, allowed user) are written to .env from Gitea Actions
repo secrets on every deploy instead of living on the server by hand.

Also switch the container's bind address to "[::]" (dual-stack) so
the app is reachable over IPv6, not just IPv4.

views/index.erb

@@ -1,6 +1,14 @@
 <div id="app">
   <header class="topbar">
     <span class="topbar__brand">MapTool</span>
+    <div class="topbar__auth">
+      <% if logged_in %>
+        <span><%= user %></span>
+        <a href="/logout">Log out</a>
+      <% else %>
+        <a href="/login">Log in</a>
+      <% end %>
+    </div>
   </header>
 
   <div class="app-body">