feat(dictionary): add Hachidori backend support

- Add backend selection, setup gating, Anki integration, and external host support
- Add launcher flags, documentation, packaging, and focused tests
- Open on-demand overlay modals on the first attempt

vendor/hachidori/extension/engine-worker-idbfs.js

@@ -0,0 +1,13 @@
+/*
+ * Engine worker for hosts with shared memory and workers but no OPFS access
+ * handles (Electron refuses them to chrome-extension:// origins): pthread
+ * Hoshidicts on the classic FS with IDBFS persistence, so imports still use the
+ * bounded worker group instead of one thread. See engine-worker-runtime.js.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+import createHoshidicts from "./vendor/hoshidicts-threaded-idbfs.mjs";
+import { startEngineWorker } from "./engine-worker-runtime.js";
+
+startEngineWorker({ createHoshidicts, storageBackend: "idbfs" });