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/blob-download.js

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+export function downloadBlob(document, blob, filename) {
+  const window = document.defaultView;
+  const url = window.URL.createObjectURL(blob);
+  try {
+    const anchor = document.createElement("a");
+    anchor.href = url;
+    anchor.download = filename;
+    anchor.click();
+  } catch (error) {
+    window.URL.revokeObjectURL(url);
+    throw error;
+  }
+  window.setTimeout(() => window.URL.revokeObjectURL(url), 60_000);
+}