- Check error returns from deferred Close calls (errcheck)
- Replace deprecated io/ioutil with os in mail.go (SA1019)
- Simplify boolean assignment in common/config.go (QF1007)
- Apply De Morgan's law in folder_recursive.go (QF1001)

Add utility.CheckClose so deferred Close calls return their error to
the caller instead of being silently discarded. Use it in the file
upload, download, and credential cache paths; the test helper reports
close errors via t.Error since it has no error return.

Previously the tests failed with "username and password are required"
when run without env vars. Skip instead so go test ./... passes in
environments without Proton credentials (e.g. CI lint-only runs).

Set the new NameSignatureEmail field on MoveLinkReq and stop sending
NodePassphraseSignature/SignatureEmail. Per the official Proton Drive
Windows client (MoveLinkParameters.cs) the latter pair are only sent
when moving an anonymously-signed node; for ordinary user-owned nodes
they must be omitted so the server preserves the existing signature on
the link.

Bump go-proton-api for the new MoveLinkReq schema.

Fixes rclone/rclone#8512

Previously the bridge built its proton.Manager with only WithAppVersion
and WithUserAgent, so consumers had no way to plug their own
http.RoundTripper into the underlying go-proton-api / resty client.

Add a Transport field to common.Config and forward it via
proton.WithTransport when non-nil. This lets callers wire in their own
transport.

Add a Logger interface (and matching Config.Logger field) whose method
set lines up with resty.Logger and go-proton-api's WithLogger option,
so callers can plug in a single adapter that captures all bridge log
output along with HTTP-layer warnings emitted by go-proton-api.

Convert the active stdlib log.Println / log.Printf call sites in
cache.go, drive.go, file_download.go, folder_recursive.go, mail.go and
common/user.go to use the configured logger via Config.GetLogger,
which falls back to the stdlib log package when no Logger is set so
existing callers see the same behaviour.

Downgrade the four log.Fatalln sites in cache.go and mail.go: a
library has no business calling os.Exit. The cache assertions become
logged errors (the offending branches were already unreachable in
practice), and the unsupported-recipient case in sendDraft now returns
a new ErrUnsupportedRecipientType.

See: rclone/rclone#9377