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/providers/base.py

@@ -9,7 +9,7 @@ from __future__ import annotations
 
 from collections.abc import AsyncIterator
 from dataclasses import dataclass
-from datetime import datetime
+from datetime import date, datetime
 from typing import Protocol
 
 
@@ -71,9 +71,25 @@ class MailProvider(Protocol):
     async def list_mailboxes(self) -> list[RemoteMailbox]: ...
 
     def backfill(
-        self, mailbox: str, *, newest_first: bool = True, batch_size: int = 200
+        self,
+        mailbox: str,
+        *,
+        newest_first: bool = True,
+        batch_size: int = 200,
+        since: date | None = None,
+        before: date | None = None,
     ) -> AsyncIterator[list[RemoteMessage]]:
-        """Yields batches until the mailbox is exhausted. Headers+body in one pass."""
+        """Yields batches until the mailbox is exhausted. Headers+body in one pass.
+        ``since``/``before`` bound the import by internal date (SPEC §12 M2)."""
+        ...
+
+    async def message_count(self, mailbox: str, *, since: date | None = None) -> int:
+        """Number of messages to import (progress denominator); whole mailbox or
+        the ``since`` window."""
+        ...
+
+    async def newest_uid(self, mailbox: str) -> int:
+        """Current high-water UID; used by the ``new`` window to skip history."""
         ...
 
     async def poll(