feat(identity): interactive topographic login background

[jaavid]

Summary

• add the supplied topographic artwork as an optimized WebP login background
• add subtle pointer-driven parallax with inertial smoothing
• add a restrained pointer-following radial highlight
• keep the effect static on touch devices and for prefers-reduced-motion
• make the Keycloak authentication card more translucent so the background remains visible
• expose the image through VITE_IDENTITY_BACKGROUND for future branding overrides

Notes

The interaction is applied at the shared KcPage level, so it is consistent across login, registration, password reset, OTP/TOTP, update-password, and other Keycloak authentication pages.

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