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/api/schemas.py

@@ -29,6 +29,7 @@ class ThreadSummary(BaseModel):
 class ThreadListOut(BaseModel):
     threads: list[ThreadSummary]
     cursor: str | None = None
+    total: int = 0
 
 
 class AttachmentOut(BaseModel):
@@ -54,6 +55,8 @@ class MessageEnvelope(BaseModel):
     is_starred: bool
     has_attachments: bool
     has_remote_images: bool
+    # The sender is on the user's trusted list, so remote images load without a prompt.
+    images_allowed: bool = False
     size_bytes: int
     snippet: str
     # Other accounts this same delivery also landed in (collapsed duplicates).
@@ -84,6 +87,9 @@ class AccountOut(BaseModel):
     color: str
     state: str
     signature: str = ""
+    sync_window: str = "all"
+    backfill_total: int = 0
+    backfill_done: int = 0
 
 
 class SearchThreadHit(BaseModel):