BUG: Unchecked qp.Close() errors in sender multipart composition

[andrinoff]

Describe the bug

In sender.go (lines 220, 360, 373, 804, 816, 830), multiple Close() calls on quoted-printable writers and multipart writers don't check the returned error. A failed Close() means the MIME part wasn't properly terminated, resulting in malformed emails.

To reproduce

1. Send an email under memory pressure where write buffers can't flush
2. Email sends with malformed MIME structure

Expected behavior

All Close() calls should check and propagate errors. Use defer with error capture where appropriate.

[floatpanebot]

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