fix: move offline SVG export from buttons() to right_page()

The buttons() plugin-template-extension hook was registered for
dcim.rack and rendered without error, but the button HTML never
appeared on the rendered rack detail page in production — NetBox's
Rack detail template apparently doesn't wire the buttons() slot into
its controls row. right_page() is confirmed working for this plugin
already (the device rack-width panel uses it), so the export links
now render there as a small card instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

netbox_utilities/template_content.py

@@ -155,12 +155,12 @@ class ConnectionVLANContent(PluginTemplateExtension):
 class RackUtilitiesContent(PluginTemplateExtension):
     models = ["dcim.rack"]
 
-    def buttons(self):
+    def right_page(self):
         if not offline_svg_export_enabled():
             return ""
         rack = self.context["object"]
         return self.render(
-            "netbox_utilities/rack_offline_svg_buttons.html",
+            "netbox_utilities/rack_offline_svg_panel.html",
             {"rack": rack},
         )