OAuth token save/load/clear now use the system keyring (GNOME Keyring,
KDE Wallet) as primary storage, with the existing AES-256-GCM
encrypted file as automatic fallback.
On first load from file, the token is migrated to the keyring
automatically for future use.
The previous commit enabled the keyring backend but only wired it
for email+password credentials (no longer used). This wires it
for the OAuth token which is the active auth method.
cc
vicrodh
241dcf0814