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/json-value.js

@@ -0,0 +1,24 @@
+/*
+ * Exact structural comparison for storage values shared by the service worker
+ * and engine worker transaction boundaries.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+export function sameJsonValue(left, right) {
+  if (left === right) return true;
+  if (Array.isArray(left) || Array.isArray(right)) {
+    return Array.isArray(left)
+      && Array.isArray(right)
+      && left.length === right.length
+      && left.every((value, index) => sameJsonValue(value, right[index]));
+  }
+  if (!left || !right || typeof left !== "object" || typeof right !== "object") {
+    return false;
+  }
+  const leftKeys = Object.keys(left);
+  const rightKeys = Object.keys(right);
+  return leftKeys.length === rightKeys.length
+    && leftKeys.every((key) =>
+      Object.hasOwn(right, key) && sameJsonValue(left[key], right[key]));
+}