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/routes/settings/Settings.tsx

@@ -225,6 +225,10 @@ function AccountsSection() {
     mutationFn: (id: string) => api.deleteAccount(id),
     onSuccess: () => queryClient.invalidateQueries({ queryKey: ["accounts"] }),
   });
+  const downloadAll = useMutation({
+    mutationFn: (id: string) => api.updateAccount(id, { sync_window: "all" }),
+    onSuccess: () => queryClient.invalidateQueries({ queryKey: ["accounts"] }),
+  });
 
   const close = () => {
     setEditing(null);
@@ -257,8 +261,25 @@ function AccountsSection() {
                   <span className="text-xs text-app-faint">
                     {t("settings.syncState", { state: a.state })}
                   </span>
+                  {a.sync_window !== "all" && (
+                    <>
+                      <span className="text-xs text-app-faint">·</span>
+                      <span className="text-xs text-app-faint">
+                        {t("settings.syncedPartially")}
+                      </span>
+                    </>
+                  )}
                 </span>
                 <span className="flex items-center gap-3">
+                  {a.sync_window !== "all" && (
+                    <button
+                      className="text-xs text-app-accent disabled:opacity-50"
+                      disabled={downloadAll.isPending || a.state === "backfilling"}
+                      onClick={() => downloadAll.mutate(a.id)}
+                    >
+                      {t("settings.downloadAllMail")}
+                    </button>
+                  )}
                   <button
                     className="text-xs text-app-accent"
                     onClick={() => setEditing({ id: a.id, mode: "signature" })}