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/tests/indexer/test_mime.py

@@ -49,6 +49,17 @@ def test_parse_basic_headers_and_body() -> None:
     assert parsed.snippet == "plain body"
 
 
+def test_undecodable_header_bytes_scrubbed_to_valid_utf8() -> None:
+    # A raw 8-bit byte the parser can't decode surfaces as a lone surrogate; it must
+    # be scrubbed so the value is valid UTF-8 for Postgres text/JSONB storage.
+    raw = _build(subject="Fakt\udce9ra", from_addr="B\udce9ta <b@example.com>")
+    parsed = parse_message(raw)
+    parsed.subject.encode("utf-8")  # would raise on a lone surrogate
+    parsed.from_addr[0]["name"].encode("utf-8")
+    assert "\udce9" not in parsed.subject
+    assert "�" in parsed.subject
+
+
 def test_references_include_in_reply_to_in_order() -> None:
     parsed = parse_message(_build(references="<a@x.com> <b@x.com>", in_reply_to="<b@x.com>"))
     # References first, then In-Reply-To, deduped