Gmail label sync + no-dup backfill, live label/UI updates, account-delete cleanup, Sync-SPAM toggle, sidebar logo

Gmail sync (SPEC: Gmail sync):
- Provider fetches X-GM-LABELS when X-GM-EXT-1 is advertised; parses custom labels
  (system labels dropped, mUTF-7 decoded) onto RemoteMessage.
- Engine syncs only system folders + All Mail for Gmail, skips every label folder,
  and dedupes the All Mail pass by Message-ID — kills the INBOX/All-Mail duplication.
- Custom labels ride the message.new event; the labels plugin subscribes and maps
  them to chips as "<name> [Gmail]" (per-message + thread aggregate + known set),
  so labels fill in gradually with the sync window. Non-ASCII labels decode right.

Labels self-healing (message-driven reconcile):
- Per-message keys are the source of truth; t: aggregates and _labels are derived.
  Reconcile drops labels of deleted messages, regroups survivors by their current
  thread (fixing JWZ thread merges), rewrites changed t: keys, and rebuilds _labels
  from live messages — keeping user labels, dropping orphaned Gmail ones.
- New SDK reads: sdk.threads.existing / sdk.messages.threads_of. Sidebar refreshes
  live via a plugin.labels.changed WS event on new labels.

Account deletion: prune threads left empty after the cascade (they were user-scoped,
so only messages cascaded) + de-list the account from surviving cross-account threads.

Sync SPAM: opt-in per-account sync_spam (migration 0015, default off) — the engine
skips the Junk folder in backfill and poll unless enabled; wizard checkbox added.

Robustness / UX:
- WebSocket auto-reconnects with backoff and resyncs on (re)connect, and account
  add/delete invalidates threads/labels — no more hard refresh for live updates.
- Backfill banner + sidebar ring also require done < total, so a completed account
  can't leave a stuck "X of X".
- Scrub lone UTF-16 surrogates from parsed headers/bodies so bad messages no longer
  crash the backfill on Postgres text/JSONB.
- Ultimo signet logo in the Mail/Calendar/Contacts sidebar header (and at the top of
  the collapsed rail), via the shared plugin-ui.css + MailLayout.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U72FZiF1W1pb8XxyWBTREA

backend/app/accounts/service.py

@@ -4,14 +4,14 @@ from __future__ import annotations
 
 import uuid
 
-from sqlalchemy import delete, select
+from sqlalchemy import delete, func, select, update
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.accounts.schemas import CreateAccountRequest
 from app.accounts.types import ServerConfig
 from app.indexer import blobstore
-from app.models import Account, Attachment, Message
+from app.models import Account, Attachment, Message, Thread
 from app.security.crypto import encrypt_secret
 
 
@@ -40,6 +40,7 @@ async def create_account(
         discovery_src="manual",
         state="backfilling",
         sync_window=req.sync_window,
+        sync_spam=req.sync_spam,
     )
     session.add(account)
     try:
@@ -81,6 +82,23 @@ async def delete_account(
     await session.execute(delete(Account).where(Account.id == account_id))
     await session.flush()
 
+    # Threads are user-scoped and can span accounts (Thread.account_ids), so the
+    # account cascade only removes its messages — not the threads. Clean up: drop
+    # threads left with no messages (else they linger as empty rows the unified list
+    # and label views still show but can't open), and de-list the account from
+    # threads that survive on another account's messages.
+    await session.execute(
+        delete(Thread).where(
+            Thread.user_id == user_id,
+            ~select(Message.id).where(Message.thread_id == Thread.id).exists(),
+        )
+    )
+    await session.execute(
+        update(Thread)
+        .where(Thread.user_id == user_id, Thread.account_ids.contains([account_id]))
+        .values(account_ids=func.array_remove(Thread.account_ids, account_id))
+    )
+
     # Deregister from sync:live and drop any lease so the owning sync worker
     # stops the account's runner on its next renewal (refactor §2 fencing).
     from app.events import get_event_bus