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-digest.js

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+export async function ankiDigest(bytes) {
+  const hash = new Uint8Array(await globalThis.crypto.subtle.digest("SHA-256", bytes));
+  return [...hash].map(byte => byte.toString(16).padStart(2, "0")).join("");
+}