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/indexer/mime.py

@@ -37,6 +37,7 @@ class ParsedEmail:
     from_addr: list[dict[str, str]]
     to_addr: list[dict[str, str]]
     cc_addr: list[dict[str, str]]
+    bcc_addr: list[dict[str, str]]
     date: datetime | None
     text_body: str
     html_body: str | None
@@ -140,6 +141,7 @@ def parse_message(raw: bytes) -> ParsedEmail:
         from_addr=_addresses(msg, "From"),
         to_addr=_addresses(msg, "To"),
         cc_addr=_addresses(msg, "Cc"),
+        bcc_addr=_addresses(msg, "Bcc"),
         date=date,
         text_body=text_body or "",
         html_body=html_body,