Fix server back links in panel theme

[dicnunz]

Summary

• route server-detail Back to Servers links through the panel wrapper
• keep the links consistent with other panel theme navigation
• add a regression check for the server-detail template route helper

Closes #188

Tests

• git diff --check
• rg -n "path('servers')" themes/default/panel/server/server.html.twig || true
• rg -n "path('panel', { routeName: 'servers' })" themes/default/panel/server/server.html.twig src/Core/Tests/Integration/Controller/ServerControllerTest.php

Could not run PHPUnit locally because this environment does not have php installed (zsh: command not found: php).