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

frontend/src/i18n/en.ts

@@ -49,6 +49,9 @@ export const en = {
   "nav.spam": "Spam",
   "nav.trash": "Trash",
   "nav.search": "Search mail…",
+  "search.save": "Save this search",
+  "search.saved": "Saved searches",
+  "search.saveNamePrompt": "Name this saved search:",
   "nav.storageQuota": "Storage used",
   "nav.refresh": "Refresh all folders",
 
@@ -176,11 +179,22 @@ export const en = {
   "wizard.startReading": "Start reading now",
   "wizard.manualConfig": "Enter settings manually",
   "wizard.displayName": "Display name",
+  "wizard.syncWindow": "How much mail to sync",
+  "wizard.syncWindow.all": "All mail",
+  "wizard.syncWindow.all.hint": "Imports the last 30 days first, then the rest in the background.",
+  "wizard.syncWindow.3months": "Last 3 months only",
+  "wizard.syncWindow.3months.hint": "Older mail stays on the server until you download it.",
+  "wizard.syncWindow.new": "New mail only",
+  "wizard.syncWindow.new.hint":
+    "Nothing from before now — download older mail anytime from account settings.",
 
   "settings.title": "Settings",
   "settings.profile": "Profile",
   "settings.security": "Security",
   "settings.accounts": "Accounts",
+  "settings.downloadAllMail": "Sync remaining mail",
+  "settings.syncedPartially": "Synced partially",
+  "backfill.importing": "Importing messages… {done} of {total}",
   "settings.appearance": "Appearance",
   "settings.handle": "Handle",
   "settings.recoveryEmail": "Recovery email",