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/app/accounts/schemas.py

@@ -3,9 +3,13 @@
 from __future__ import annotations
 
 import uuid
+from typing import Literal
 
 from pydantic import BaseModel, Field
 
+# How much history to import at account setup (SPEC §12 M2).
+SyncWindow = Literal["all", "3months", "new"]
+
 
 class ServerConfigIn(BaseModel):
     host: str
@@ -56,12 +60,14 @@ class CreateAccountRequest(BaseModel):
     smtp: ServerConfigIn
     username: str | None = None
     password: str = Field(min_length=1)
+    sync_window: SyncWindow = "all"
 
 
 class UpdateAccountRequest(BaseModel):
     display_name: str | None = None
     color: str | None = None
     signature: str | None = None
+    sync_window: SyncWindow | None = None
     # Connection/credential changes require the password (re-validated + re-encrypted).
     imap: ServerConfigIn | None = None
     smtp: ServerConfigIn | None = None
@@ -76,5 +82,8 @@ class AccountDetailOut(BaseModel):
     state: str
     state_detail: str
     signature: str
+    sync_window: str
+    backfill_total: int
+    backfill_done: int
     imap_config: dict[str, object]
     smtp_config: dict[str, object]