Restore frontend router after loading member form libraries - #5350

{layout="site:_layouts/wrapper"}

{exp:member:edit_profile}
    {field:company_name}
{/exp:member:edit_profile}

{var_licenses-table_styles_2024}
When this unresolved variable was placed in <head>, the browser moved the text into <body>, causing it to appear visibly above the site header.

Other code executed later in the request—including add-ons and extensions—could also choose the wrong behavior if it checks the router to distinguish frontend and Control Panel requests.
The fix records the original router class, temporarily switches to cp, loads the required libraries, and then restores the original value. It restores rather than hard-codes ee, preserving whichever context initiated the request.

When this unresolved variable was placed in <head>, the browser moved the text into <body>, causing it to appear visibly above the site header.

Other code executed later in the request—including add-ons and extensions—could also choose the wrong behavior if it checks the router to distinguish frontend and Control Panel requests.

The fix records the original router class, temporarily switches to cp, loads the required libraries, and then restores the original value. It restores rather than hard-codes ee, preserving whichever context initiated the request.

## Longer-term fix

Frontend Member forms should not need to mutate the global router to load field rendering and JavaScript dependencies. The shared form functionality should be extracted from the CP-only library or made explicitly usable in a frontend context.

That would remove the temporary `cp` masquerade entirely. The save-and-restore change in this PR is a safe short-term fix while preserving the existing library behavior.