fix(mail): render single newlines in draft bodies as line breaks

[piekstra]

Fixes #32

gro mail draft rendered markdown bodies with CommonMark soft breaks, so a single newline became a space and a two-line sign-off (Thanks, then a name) arrived joined on one line. The renderer now uses goldmark's WithHardWraps, so a single newline becomes <br> while a blank line still separates paragraphs. This matches how the quoted-reply path in the same command already treats newlines.

TestRenderMarkdown_SingleNewlineIsALineBreak pins both halves: the sign-off keeps its line break and the blank line still yields two paragraphs. It fails on main and passes with the change; make lint is clean.

[monit-reviewer]

Automated PR Review

Reviewed commit: d18e7a0cb46e
Profile: claude-monit-reviewer - Posting as: monit-reviewer

Summary

Reviewer	Findings
go:implementation-tests	0

Reviewer Coverage

• go:implementation-tests — complete (broad); skipped: none; constraints: none

Inspected files (2)

• internal/cmd/mail/draft.go
• internal/cmd/mail/draft_test.go

0 PR discussion threads considered. 0 summarized; 0 resolved.

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Completed in 1m 42s | ~$0.83 (est.) | claude-sonnet-5 | cr 0.10.311

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