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/migrations/versions/0015_account_sync_spam.py

@@ -0,0 +1,28 @@
+"""add accounts.sync_spam
+
+Per-account toggle for whether the Spam/Junk folder is synced. Off by default —
+most users don't want spam mirrored into the app; they can opt in at setup.
+
+Revision ID: 0015
+Revises: 0014
+Create Date: 2026-08-05
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "0015"
+down_revision: str | None = "0014"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TABLE accounts ADD COLUMN sync_spam boolean NOT NULL DEFAULT false")
+
+
+def downgrade() -> None:
+    op.execute("ALTER TABLE accounts DROP COLUMN IF EXISTS sync_spam")