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/audio-sources.js

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Browser-native source rules adapted from GSM PR #549's hoshidicts_audio.py
+// and hoshidicts_audio_profile.py. Configuration remains global reader options.
+
+function httpUrl(value) {
+  if (typeof value !== "string" || /[\u0000-\u001f]/u.test(value)) {
+    throw new Error("Audio URLs must be absolute HTTP(S) URLs.");
+  }
+  const url = new URL(value);
+  if (!["http:", "https:"].includes(url.protocol) || url.username || url.password || url.port === "0") {
+    throw new Error("Audio URLs must use HTTP(S) without a username or password.");
+  }
+  if (/[{}]/u.test(url.host)) throw new Error("Audio URL placeholders cannot appear in the host.");
+  return url.href;
+}
+
+function encodeValue(value) {
+  return encodeURIComponent(value).replace(/[!'()*]/gu, char => `%${char.codePointAt(0).toString(16).toUpperCase()}`);
+}
+
+export function audioSourceUrl(template, { expression, reading }) {
+  httpUrl(template);
+  const encodedTerm = encodeValue(expression);
+  const values = { term: encodedTerm, expression: encodedTerm, reading: encodeValue(reading), language: "ja" };
+  return httpUrl(template.replace(/\{([^{}]*)\}/gu, (match, key) => Object.hasOwn(values, key) ? values[key] : match));
+}
+
+export function parseAudioSourceList(value) {
+  if (value?.type !== "audioSourceList" || !Array.isArray(value.audioSources)
+      || Object.keys(value).some(key => !["type", "audioSources"].includes(key))) {
+    throw new Error("The audio provider returned an invalid Yomitan audioSourceList.");
+  }
+  return value.audioSources.map(candidate => {
+    if (!candidate || typeof candidate.url !== "string" || (candidate.name !== undefined && typeof candidate.name !== "string")
+        || Object.keys(candidate).some(key => !["url", "name"].includes(key))) {
+      throw new Error("The audio provider returned an invalid pronunciation candidate.");
+    }
+    return { url: httpUrl(candidate.url), name: candidate.name ?? "" };
+  });
+}