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/migrations/versions/0012_account_sync_window.py

@@ -0,0 +1,35 @@
+"""add accounts.sync_window + backfill progress counters
+
+Lets the user choose at account setup how much history to import (SPEC §12 M2):
+``all`` (default; recent 30d first, then the rest), ``3months`` (windowed), or
+``new`` (no backfill — only mail arriving after setup). ``backfill_total`` /
+``backfill_done`` drive the initial-sync progress UX; total 0 means "nothing to
+import" (the ``new`` window, or an empty account).
+
+Revision ID: 0012
+Revises: 0011
+Create Date: 2026-07-23
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "0012"
+down_revision: str | None = "0011"
+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_window text NOT NULL DEFAULT 'all'")
+    op.execute("ALTER TABLE accounts ADD COLUMN backfill_total integer NOT NULL DEFAULT 0")
+    op.execute("ALTER TABLE accounts ADD COLUMN backfill_done integer NOT NULL DEFAULT 0")
+
+
+def downgrade() -> None:
+    op.execute("ALTER TABLE accounts DROP COLUMN IF EXISTS backfill_done")
+    op.execute("ALTER TABLE accounts DROP COLUMN IF EXISTS backfill_total")
+    op.execute("ALTER TABLE accounts DROP COLUMN IF EXISTS sync_window")