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/migrations/versions/0009_account_signature.py

@@ -0,0 +1,28 @@
+"""add accounts.signature
+
+Per-account HTML signature, configured in Settings → Accounts and inserted into
+outgoing messages by the composer.
+
+Revision ID: 0009
+Revises: 0008
+Create Date: 2026-07-18
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "0009"
+down_revision: str | None = "0008"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TABLE accounts ADD COLUMN signature text NOT NULL DEFAULT ''")
+
+
+def downgrade() -> None:
+    op.execute("ALTER TABLE accounts DROP COLUMN IF EXISTS signature")