Implement prepareForValidation method for NGINX

[pashdown]

Under NGINX, the customer statistics would always return to "bits" and "day" regardless of menu selections. I don't know if this problem exists under Apache. Note that I was led to this fix by claude.ai.

PR template - remove this line and edit below

[BF] Summary of fix - fixes [inex|islandbridgenetworks]/IXP-Manager#x

[NF] New feature summary - closes [inex|islandbridgenetworks]/IXP-Manager#x

Longer description

In addition to the above, I have:

• [ ] ensure unit tests all run without error
• [ ] ran pslam and corrected any static analysis issues
• [ ] ensured all relevant template output is escaped to avoid XSS attached with <?= $t->ee( $data ) ?> or equivalent.
• [ ] ensured appropriate checks against user privilege / resources accessed
• [ ] API calls (particular for add/edit/delete/toggle) are not implemented with GET and use CSRF tokens to avoid CSRF attacks