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/chrome-offscreen.js

@@ -0,0 +1,44 @@
+// Chrome MV3 lifecycle for the shared offscreen engine document.
+// Firefox imports this shared module but never calls its guarded Chrome path.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+import { extensionApi as chrome } from "./browser-api.js";
+
+let creating = null;
+
+export function chromeOffscreenSupported() {
+  return typeof chrome.runtime.getContexts === "function"
+    && typeof chrome.offscreen?.createDocument === "function";
+}
+
+async function offscreenExists(url) {
+  const contexts = await chrome.runtime.getContexts({
+    contextTypes: ["OFFSCREEN_DOCUMENT"],
+    documentUrls: [chrome.runtime.getURL(url)],
+  });
+  return contexts.length > 0;
+}
+
+async function createOffscreen(url) {
+  try {
+    await chrome.offscreen.createDocument({
+      url,
+      reasons: ["DOM_SCRAPING", "AUDIO_PLAYBACK", "DISPLAY_MEDIA"],
+      justification:
+        "Runs the dictionary engine and pronunciation audio, and owns explicitly started local display capture across control-page closure.",
+    });
+  } catch (error) {
+    // Another extension context may have won the race; only a genuine absence
+    // is a failure.
+    if (!(await offscreenExists(url))) throw error;
+  } finally {
+    creating = null;
+  }
+}
+
+export async function ensureChromeOffscreen(url) {
+  if (!chromeOffscreenSupported()) return;
+  if (await offscreenExists(url)) return;
+  if (creating === null) creating = createOffscreen(url);
+  await creating;
+}