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/external-links.js

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Shared by classic content scripts and the module service worker.
+(function () {
+  "use strict";
+
+  function normaliseExternalUrl(value) {
+    if (typeof value !== "string") return null;
+    if (/[\u0000-\u001f\u007f]/u.test(value)) return null;
+    const source = value.trim();
+    if (!/^https?:\/\//iu.test(source)) return null;
+    try {
+      const url = new URL(source);
+      return (url.protocol === "http:" || url.protocol === "https:") && !url.username && !url.password
+        ? url.href : null;
+    } catch {
+      return null;
+    }
+  }
+
+  // GSM popup links use %w for the word and %s for the sentence. Reading is
+  // useful to dictionary providers too; encode each value before substitution.
+  function expandCustomLinkUrl(template, { word = "", reading = "", sentence = "" } = {}) {
+    if (typeof template !== "string") return null;
+    const values = { w: word, r: reading, s: sentence };
+    const expanded = template.replace(/%([wrs])/gu, (_, marker) => encodeURIComponent(String(values[marker])));
+    return normaliseExternalUrl(expanded);
+  }
+
+  globalThis.HDExternalLinks = { normaliseExternalUrl, expandCustomLinkUrl };
+})();