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/tests/api/test_api.py

@@ -234,6 +234,14 @@ async def test_attachments_list_and_zip(api: ApiCtx) -> None:
     assert archive.read("notes.txt") == b"hello attachment"
 
 
+async def test_quota_reports_usage_and_default(api: ApiCtx) -> None:
+    resp = await api.client.get("/api/settings/quota")
+    assert resp.status_code == 200
+    body = resp.json()
+    assert body["quota_bytes"] == 5 * 1024**3  # default 5 GiB
+    assert body["used_bytes"] > 0  # seeded messages have a size
+
+
 async def test_thread_detail_messages_ascending(api: ApiCtx) -> None:
     # find the kickoff thread (2 messages)
     resp = await api.client.get("/api/threads")