The Jellyseerr -> Seerr rebrand renamed the settings property
(jellyseerrServerUrl -> seerrServerUrl) and the MMKV storage keys
(JELLYSEERR_USER/COOKIES -> SEERR_USER/COOKIES). Without migration,
users who configured Seerr before the rename lose their server URL and
are silently logged out on update.
Adds one-time migrations: loadSettings copies the legacy property, and
useSeerr migrates the storage keys at module load before the user atom
is read.