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/components/thread/ThreadView.tsx

@@ -120,6 +120,9 @@ function MessageItem({
               {message.cc_addr.length > 0 && (
                 <span className="text-app-faint"> · cc: {formatList(message.cc_addr)}</span>
               )}
+              {message.bcc_addr.length > 0 && (
+                <span className="text-app-faint"> · bcc: {formatList(message.bcc_addr)}</span>
+              )}
             </span>
             <span className="shrink-0 text-xs text-app-faint">{new Date(message.date).toLocaleString()}</span>
           </span>
@@ -193,7 +196,7 @@ export function ThreadView({ subject, messages, order, replyKindSignal }: Thread
 
   return (
     <div data-testid="thread-view" className="flex h-full flex-col overflow-y-auto bg-app-bg">
-      <div className="sticky top-0 z-10 flex h-[44px] items-center justify-between border-b border-app-border bg-app-surface px-4">
+      <div className="sticky top-0 z-10 flex h-[44px] shrink-0 items-center justify-between border-b border-app-border bg-app-surface px-4">
         <h2 className="truncate text-base font-semibold text-app-fg">{subject || "(no subject)"}</h2>
         <button
           className="shrink-0 text-xs text-app-accent hover:underline"