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>

frontend/src/lib/ws.ts

@@ -45,6 +45,8 @@ export function connectWs(queryClient: QueryClient): () => void {
     // Coarse invalidation: refetch affected lists (SPEC §9.2 contract).
     if (name === "sync.progress" || name.startsWith("message.") || name === "thread.updated") {
       queryClient.invalidateQueries({ queryKey: ["threads"] });
+      queryClient.invalidateQueries({ queryKey: ["quota"] });
+      queryClient.invalidateQueries({ queryKey: ["drafts-remote"] });
       if (typeof payload.thread_id === "string") {
         queryClient.invalidateQueries({ queryKey: ["thread", payload.thread_id] });
       }