Send attachments

- Compose gains an "Attach file" picker (OpenMultipleDocuments) and shows each pick as
  a removable chip; OutgoingMessage carries the picked URIs.
- On send the repository copies the picked files into the outbox message's own cache
  directory; SendWorker passes them to SmtpSender, which builds a multipart message
  (text body + a part per file via attachFile). Files are cleaned up on success/cancel.
- assemble/test/lint green, including a new SmtpSender test that sends an attachment and
  asserts GreenMail received a multipart message containing it; the compose "Attach file"
  affordance verified on the Android 17 emulator.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -16,7 +16,8 @@ experience with power-user features tucked under an **Advanced Settings** group.
 > later; **on-device new-mail
 > notifications** (no push service) with persisted settings; **instant push** via a
 > foreground **IMAP IDLE** service; **attachments** — downloaded on demand and opened in a
-> system viewer; **multiple accounts** — a unified inbox with per-account filtering; and
+> system viewer, and attach files when composing; **multiple accounts** — a unified inbox
+> with per-account filtering; and
 > **search** across cached mail. Outlook Graph send is upcoming.
 
 ## Features (target MVP)