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/firefox-background.js

@@ -0,0 +1,13 @@
+// Firefox MV2 persistent background-page entry point.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Both imports are static so every runtime listener in background.js exists
+// before this module body runs. Firefox delivers runtime.onInstalled and
+// runtime.onStartup only to listeners registered during the background page's
+// synchronous evaluation; a dynamic import would miss them and first-run setup
+// would never open. The host's message listener is attached here, after
+// background.js, and before the engine iframe that announces itself to it.
+import "./background.js";
+import { createFirefoxBackgroundHost } from "./firefox-host.js";
+
+createFirefoxBackgroundHost(document).mount();