fix(TO8-48): increase session timeout to 4h, add JS keep-alive ping

fix(TO8-48): increase session timeout to 4h, add JS keep-alive ping

Prevent BadRequestHttpException (Unable to verify your data submission)
in WriteOffsErpController caused by PHP session expiry during long form filling.

- erp24/config/web.php: add session component, timeout=14400 (4 hours)
- SiteController: add actionKeepAlive() endpoint for JS ping
- session-keep-alive.js: ping /site/keep-alive every 15 min, pause on hidden tab, skip during upload
- main.php: register session-keep-alive.js globally