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/local-audio-source.js

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+export const LOCAL_AUDIO_SOURCE_URL = "http://127.0.0.1:5050/?term={term}&reading={reading}";
+
+export function findLocalAudioSource(sources) {
+  return sources.find(source => source.type === "custom-json" && source.url === LOCAL_AUDIO_SOURCE_URL) ?? null;
+}
+
+export function createLocalAudioSource(id, url = LOCAL_AUDIO_SOURCE_URL) {
+  return { id, type: "custom-json", enabled: true, url, voice: "" };
+}