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-index-worker.js

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+import { createAnkiGateway } from "./anki.js";
+import { fetchAnkiIndex } from "./anki-index.js";
+
+self.onmessage = async ({ data: source }) => {
+  try {
+    const gateway = createAnkiGateway();
+    const invoke = (action, params, timeoutMs) =>
+      gateway.invoke(action, params, source.apiKey, timeoutMs, source.url);
+    self.postMessage({ rows: await fetchAnkiIndex(invoke, source) });
+  } catch (error) {
+    self.postMessage({ error: error.message || String(error) });
+  }
+};