Update notifications for self-hosted admins

Self-hosted admins now see an Update available banner on the Retool home page when a newer release or patch update is available for their instance.
The banner shows the available version and links directly to the relevant release notes. Admins can dismiss each notification individually — dismissals are remembered per version.
Two notification tiers appear based on what's available:
- New release — shown when a new minor or major release is available.
- New patch — shown when a patch update is available for the current release.
On airgapped instances, no outbound version check is made and the banner does not appear.