fix: remove flowed claim

[resolvicomai]

What?

Fixes #1114. Plaintext-only messages now emit Content-Type: text/plain; charset=UTF-8 without format=flowed, and both plaintext MIME paths share the same header writer. A regression test covers that the header no longer advertises flowed text while keeping quoted-printable encoding.

Why?

The sender quoted-printable encodes the body but does not implement RFC 3676 format=flowed space-stuffing or soft line break semantics. Advertising format=flowed can make conforming clients reflow plain text incorrectly, especially quoted text.

Validation:

• go test ./sender -run TestWritePlainTextPartHeadersDoesNotClaimFormatFlowed -count=1
• go test ./sender -count=1
• go test ./...
• make lint
• git diff --check

[FromSi]

lgtm

[andrinoff]

conflicts with your own PR @resolvicomai

[resolvicomai]

Thanks for flagging it. I rebased this PR on current master after #1318 merged and resolved the conflict by keeping the writeQuotedPrintable helper from #1318 plus the plaintext header helper from this PR.

Validation after rebase:

• go test ./sender
• go test ./...
• make lint
• git diff --check origin/master...HEAD

The branch is pushed again as ac4938f.