BUG: Unchecked error on quoted-printable writer Close()

[andrinoff]

Describe the bug

In sender/sender.go line 220, quotedprintable.NewWriter().Close() error is ignored:

var encBody bytes.Buffer
qp := quotedprintable.NewWriter(&encBody)
fmt.Fprint(qp, plainBody)
qp.Close()  // Error ignored
encodedBody := encBody.Bytes()

If Close() fails (write errors, flush failures), encoded body may be incomplete/corrupted but code proceeds silently.

To reproduce

1. Compose email with plain text body
2. Simulate write error during quoted-printable encoding
3. Close() fails but error not caught
4. Corrupted email sent

Expected behavior

Check Close() error:

var encBody bytes.Buffer
qp := quotedprintable.NewWriter(&encBody)
if _, err := fmt.Fprint(qp, plainBody); err != nil {
    return err
}
if err := qp.Close(); err != nil {
    return fmt.Errorf("quoted-printable encoding failed: %w", err)
}
encodedBody := encBody.Bytes()

Screenshots

N/A

Additional context

• File: sender/sender.go
• Lines: 217-221
• Severity: Medium - email body corruption
• Fix complexity: Easy - add error check