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/custom-javascript.js

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+const SCRIPT_ID = "hachidori-custom-javascript";
+let application = Promise.resolve();
+
+// Chrome MV3 `userScripts` registration. Firefox MV2 has no `userScripts`
+// with a USER_SCRIPT world, so the saved code stays inert there and Settings
+// hides the editor through HOST_CAPABILITIES.customJavaScript.
+async function apply(browser, code) {
+  let scripts;
+  try {
+    scripts = browser.userScripts;
+    await scripts.getScripts({ ids: [SCRIPT_ID] });
+  } catch {
+    return { supported: false, registered: false };
+  }
+  await scripts.unregister({ ids: [SCRIPT_ID] }).catch(() => {});
+  if (code === "") return { supported: true, registered: false };
+  await scripts.register([{
+    id: SCRIPT_ID,
+    matches: ["<all_urls>"],
+    js: [{ code }],
+    runAt: "document_idle",
+    world: "USER_SCRIPT",
+  }]);
+  return { supported: true, registered: true };
+}
+
+export function applyCustomJavaScript(browser, code) {
+  application = application.catch(() => {}).then(() => apply(browser, code));
+  return application;
+}