feat(storage): add S3 backend support and advanced upload limits

- Introduce S3-compatible storage backend support using minio-go.
- Add configuration options for local storage limits, box limits, and rate limiting.
- Implement storage backend selection (local vs S3) for anonymous and registered users.
- Add an `/admin/storage` management interface.
- Update documentation and environment examples with the new configuration variables.

backend/templates/pages/admin.html

@@ -8,6 +8,7 @@
       <a class="sidebar-link {{if eq .Data.Section "files"}}is-active{{end}}" href="/admin/files">Files</a>
       <a class="sidebar-link" href="/admin/users">Users</a>
       <a class="sidebar-link" href="/admin/settings">Settings</a>
+      <a class="sidebar-link" href="/admin/storage">Storage</a>
     </nav>
     <hr class="sidebar-sep">
     <nav class="sidebar-nav">
@@ -15,6 +16,7 @@
     </nav>
     <hr class="sidebar-sep">
     <form class="sidebar-logout" action="/admin/logout" method="post">
+      <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
       <button class="button button-outline" type="submit">Sign out</button>
     </form>
   </aside>
@@ -96,6 +98,7 @@
                 <td class="table-actions">
                   <a class="button button-outline" href="/admin/boxes/{{.ID}}/view">View</a>
                   <form action="/admin/boxes/{{.ID}}/delete" method="post">
+                    <input type="hidden" name="csrf_token" value="{{$.CSRFToken}}">
                     <button class="button button-danger" type="submit">Delete</button>
                   </form>
                 </td>