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/anki-addon.js

@@ -0,0 +1,13 @@
+// Pin the add-on independently so older extensions and vendored copies keep
+// downloading the relay they were tested with.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+export const ANKI_ADDON_FILE_NAME = "hachidori-relay.ankiaddon";
+export const ANKI_ADDON_VERSION = "0.0.4";
+export const ANKI_ADDON_URL = `https://github.com/bee-san/hachidori-anki/releases/download/v${ANKI_ADDON_VERSION}/${ANKI_ADDON_FILE_NAME}`;
+
+export async function fetchAnkiAddon(request = globalThis.fetch) {
+  const response = await request(ANKI_ADDON_URL);
+  if (!response.ok) throw new Error(`GitHub returned HTTP ${response.status}. Try again.`);
+  return response.blob();
+}