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>

app/src/main/res/values/strings.xml

@@ -40,6 +40,8 @@
     <string name="compose_subject">Subject</string>
     <string name="compose_body">Message</string>
     <string name="compose_no_account">Add an account before composing</string>
+    <string name="compose_attach">Attach file</string>
+    <string name="compose_attachment_remove">Remove attachment</string>
 
     <!-- Drafts -->
     <string name="drafts_title">Drafts</string>