M2: Meili search + saved searches, sync windows + backfill progress, threading & image fixes

Search:
- Meilisearch backend (app/search/meili.py) + factory wiring; pinned SDK 2.7
- Search-as-you-type: debounced query + Postgres prefix matching, built from
  the DB tokenizer so full email addresses match (email stays one lexeme)
- Saved searches API + sidebar virtual mailboxes; save affordance in the box

Account sync windows (SPEC §12 M2):
- Choose at setup: All (recent 30d first, then remainder), Last 3 months, New only
- Per-account sync_window + "Sync remaining mail" from settings (idempotent re-run)
- Provider windowed backfill (SINCE/BEFORE), message_count, newest_uid
- migration 0012: accounts.sync_window + backfill_total/backfill_done

Backfill progress UX:
- Floating progress card (auto-dissolves after 10s) + spinning sidebar ring that
  fills with the account colour; driven by sync.progress over WS

Thread list pagination:
- Server-side offset + accurate grand total, with filter/order in SQL; working
  prev/next across all synced mail

Reading panel:
- "Load images" one-off now actually renders images (?images=1) without trusting
  the sender; envelope carries images_allowed so a trusted sender shows no prompt

Threading:
- Subject fallback no longer merges messages that declare an explicit (different)
  parent; true siblings still group via a shared-parent reference match. Fixes
  unrelated auto-replies with the same subject landing in one thread

Backend 257 tests green, ruff + mypy --strict clean; frontend tsc/eslint/vitest clean.

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

backend/tests/api/test_api.py

@@ -378,12 +378,13 @@ async def test_accounts_list(api: ApiCtx) -> None:
     assert resp.json()[0]["email"] == "me@example.com"
 
 
-async def test_thread_pagination_cursor(api: ApiCtx) -> None:
-    first = await api.client.get("/api/threads", params={"limit": 1})
+async def test_thread_pagination_offset(api: ApiCtx) -> None:
+    first = await api.client.get("/api/threads", params={"limit": 1, "offset": 0})
     body = first.json()
     assert len(body["threads"]) == 1
-    assert body["cursor"] is not None
-    second = await api.client.get("/api/threads", params={"limit": 1, "cursor": body["cursor"]})
+    assert body["total"] >= 2  # more than one page of threads exists
+    second = await api.client.get("/api/threads", params={"limit": 1, "offset": 1})
     body2 = second.json()
     assert len(body2["threads"]) == 1
+    assert body2["total"] == body["total"]  # grand total is stable across pages
     assert body2["threads"][0]["id"] != body["threads"][0]["id"]