Two-way draft sync, signatures, sent/bcc fixes, and mail-list UI

Backend
- Sent copies: persist locally on send + periodic multi-folder CONDSTORE poll
  (Sent/Archive/Trash/Junk) so they thread and show without an IDLE-only INBOX.
- Bcc: transmitted copy omits Bcc, Sent copy keeps it; store/display bcc_addr.
- Drafts: mirror local drafts to the IMAP Drafts folder (append/replace/expunge);
  inbound sync of foreign drafts (un-threaded) with adopt-to-edit + dedup by UID.
- Per-user storage quota (5 GiB default) and manual "sync all folders" action.
- Account editing (display/color/server/password) and per-account HTML signature.
- Account view scoped to that account's inbox; new migrations 0006–0009.

Frontend
- Mail-list pagination, folder filters (All/Unread/Pinned), sort toggle, refresh.
- Aligned + widened mail-list margins; reading-pane bars fixed height.
- Rich-text composer: color palette, highlight, image; attachments; expand/pop-out.
- Signatures: auto-insert for new mail, replace on From change, reply insert.
- Restore/Delete actions per folder; drafts view merges local + remote, date-sorted.

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

backend/app/outbox/mime_build.py

@@ -49,8 +49,13 @@ def build_mime(
     draft: Draft,
     reply_to: Message | None,
     attachments: list[tuple[str, str, bytes]] | None = None,
-) -> tuple[bytes, str, list[str]]:
-    """Return ``(raw_rfc822, envelope_from, envelope_to)`` for ``draft``.
+) -> tuple[bytes, bytes, str, list[str]]:
+    """Return ``(transmit_raw, sent_raw, envelope_from, envelope_to)`` for ``draft``.
+
+    ``transmit_raw`` is sent to the SMTP recipients and carries NO ``Bcc`` header.
+    ``sent_raw`` is the copy filed in Sent; it additionally includes a ``Bcc``
+    header so the sender keeps a record of blind recipients (standard client
+    behavior). They are identical when there is no Bcc.
 
     ``attachments`` is a list of ``(filename, mime_type, data)`` uploaded via the
     compose Attach action; each is added as a file attachment part.
@@ -99,4 +104,10 @@ def build_mime(
         )
 
     envelope_to = [_email_only(a) for a in (to + cc + bcc)]
-    return msg.as_bytes(), account.email, envelope_to
+    transmit_raw = msg.as_bytes()  # no Bcc header -> safe to transmit
+    if bcc:
+        msg["Bcc"] = ", ".join(bcc)
+        sent_raw = msg.as_bytes()
+    else:
+        sent_raw = transmit_raw
+    return transmit_raw, sent_raw, account.email, envelope_to