feat: let composes carry custom frontmatter headers

README.md

@@ -8,6 +8,7 @@ A minimalist desktop client for [Smol Mail](../smolmail): one compact window —
 - Pin a server's key — paste the operator's, or connect and pin what it presents — then register a username or recall an existing address.
 - Fetch (with cancel), read, reply and send sealed mail; the inbox, the requests tier and sent copies each keep their own folder.
 - Composes closed by any sidebar trip are stashed as drafts — kept beside the store, reopened from a folder that appears only once one exists; Cancel is the only way to throw one away.
+- Custom `Key: value` frontmatter rows when writing: one line per header under the subject, validated against the frontmatter rules — the client's own fields (subject, reply-to, in-reply-to, accept) can't be shadowed, and rows survive a stash as part of the draft.
 - Accept or block a requester when their mail carried a self-certifying reply address; accept tokens attach to accepted contacts automatically.
 - Contacts with verified/tofu marks and smol:// import; your own identity opens in the same detail view, its address in the header bar with a QR share beside it, key, fingerprint and URI below.
 - Key rotation with a confirm-first step, server pin management, a network timeout, and gsmol backup export/import that never contains the seed.