Messages size at once; quoted inline images travel inside the reply

The message frame is set up as soon as its document is parsed instead of at
its load event, which waits for every image (remote ones included): it no
longer sits at its first height until the slowest image arrives.

A reply quoted an original's inline (cid:) images as this server's own URLs,
and they were sent like that — broken for the recipient, and mangled copies
("http:///api/…") made browsers look up a host named "api". Outgoing mail
(and the IMAP Drafts copy) now carries them as inline parts with new
Content-IDs; one that can no longer be had is left out. Shown messages load
such URLs from this server (sanitizer v5).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/outbox/mime_build.py

@@ -14,6 +14,7 @@ from email.utils import format_datetime, make_msgid
 import nh3
 
 from app.models import Account, Draft, Message
+from app.outbox.inline_images import InlinePart
 
 
 def _html_to_text(html: str) -> str:
@@ -49,6 +50,9 @@ def build_mime(
     draft: Draft,
     reply_to: Message | None,
     attachments: list[tuple[str, str, bytes]] | None = None,
+    *,
+    html: str | None = None,
+    inline: list[InlinePart] | None = None,
 ) -> tuple[bytes, bytes, str, list[str]]:
     """Return ``(transmit_raw, sent_raw, envelope_from, envelope_to)`` for ``draft``.
 
@@ -59,13 +63,17 @@ def build_mime(
 
     ``attachments`` is a list of ``(filename, mime_type, data)`` uploaded via the
     compose Attach action; each is added as a file attachment part.
+
+    ``html`` replaces the draft's HTML when given (images already turned into
+    ``cid:`` references by app/outbox/inline_images.py), and ``inline`` are the
+    image parts those references name, sent alongside the HTML.
     """
     payload = draft.payload
     to = _addr_list(payload.get("to"))
     cc = _addr_list(payload.get("cc"))
     bcc = _addr_list(payload.get("bcc"))
     subject = str(payload.get("subject", ""))
-    html = str(payload.get("html", "") or "")
+    html = html if html is not None else str(payload.get("html", "") or "")
     text = str(payload.get("text", "") or "")
 
     msg = EmailMessage()
@@ -91,6 +99,19 @@ def build_mime(
     if html:
         msg.set_content(text or _html_to_text(html))
         msg.add_alternative(html, subtype="html")
+        if inline:
+            html_part = next(p for p in msg.walk() if p.get_content_type() == "text/html")
+            assert isinstance(html_part, EmailMessage)
+            for part in inline:
+                maintype, _, subtype = part.mime_type.partition("/")
+                html_part.add_related(
+                    part.data,
+                    maintype=maintype or "image",
+                    subtype=subtype or "png",
+                    cid=f"<{part.cid}>",
+                    filename=part.filename,
+                    disposition="inline",
+                )
     else:
         msg.set_content(text)