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/speech.js

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Chrome may return an empty list until its first voiceschanged event. Keep
+// this wait inside the caller's existing deadline and cancellation.
+function waitForSpeechVoices(speech, signal) {
+  signal.throwIfAborted();
+  return new Promise((resolve, reject) => {
+    function clean() {
+      speech.removeEventListener("voiceschanged", changed);
+      signal.removeEventListener("abort", aborted);
+    }
+    function changed() { clean(); resolve(speech.getVoices()); }
+    function aborted() { clean(); reject(signal.reason); }
+    speech.addEventListener("voiceschanged", changed, { once: true });
+    signal.addEventListener("abort", aborted, { once: true });
+    // The voice service can finish loading between the initial query and the
+    // listener registration; checking again avoids missing that transition.
+    if (speech.getVoices().length) changed();
+  });
+}
+
+export async function resolveSpeech(window, source, term, signal) {
+  const speech = window.speechSynthesis;
+  let voices = speech.getVoices();
+  if (!voices.length) voices = await waitForSpeechVoices(speech, signal);
+  signal.throwIfAborted();
+  if (voices.length === 0) throw new Error("No speech voices are available in this browser.");
+  const text = source.type === "text-to-speech-reading" ? term.reading || term.expression : term.expression;
+  const utterance = new window.SpeechSynthesisUtterance(text);
+  utterance.lang = "ja-JP";
+  const japanese = voices.filter(voice => /^ja(?:[-_]|$)/i.test(voice.lang));
+  const voice = source.voice
+    ? voices.find(value => value.voiceURI === source.voice || value.name === source.voice)
+    : japanese.find(value => /^Google\b/i.test(value.name))
+      || japanese.find(value => value.default) || japanese[0];
+  if (source.voice && !voice) {
+    throw new Error("The selected speech voice is no longer available. Choose another voice in Audio Settings.");
+  }
+  if (!voice) {
+    throw new Error("No Japanese speech voice is available. Install a Japanese voice or add an audio provider in Audio Settings.");
+  }
+  utterance.voice = voice;
+  return {
+    speech,
+    utterance,
+    candidate: { name: voice.name, text, voice: source.voice, index: 0 },
+  };
+}