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/search/test_search_api.py

@@ -150,6 +150,24 @@ async def test_free_text(seeded: Ctx) -> None:
     assert await _search(seeded, "lunch") == 1
 
 
+async def test_free_text_prefix_matches_while_typing(seeded: Ctx) -> None:
+    # Search-as-you-type: a partial word finds the message before it is complete.
+    assert await _search(seeded, "reven") == 1  # -> "revenue"
+    assert await _search(seeded, "lun") == 1  # -> "lunch"
+    # Prefix is anchored at word start, not a substring match.
+    assert await _search(seeded, "enue") == 0
+    # Multiple partial words are ANDed as prefixes.
+    assert await _search(seeded, "quart reven") == 1  # "Quarterly revenue"
+    assert await _search(seeded, "quart lun") == 0
+
+
+async def test_free_text_full_email_address(seeded: Ctx) -> None:
+    # Postgres keeps an email as a single lexeme; searching the *whole* address
+    # must still match (regression: prefix-splitting on word chars broke this).
+    assert await _search(seeded, "bob@corp.com") == 1
+    assert await _search(seeded, "bob@corp.co") == 1  # prefix while still typing
+
+
 async def test_from_operator(seeded: Ctx) -> None:
     assert await _search(seeded, "from:bob") == 1
     assert await _search(seeded, "from:corp.com") == 3  # all three share the domain