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/visual-novel.js

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+(function () {
+  "use strict";
+  const backgrounds = [
+    { file: "preview-background.webp" },
+    { file: "preview-background-2.webp" },
+    { file: "preview-background-3.webp" },
+    { file: "preview-background-4.webp", dark: true },
+    { file: "preview-background-5.webp" },
+    { file: "preview-background-6.webp", dark: true },
+  ];
+
+  function initialize(scene) {
+    let index = crypto.getRandomValues(new Uint32Array(1))[0] % backgrounds.length;
+    const show = () => {
+      const background = backgrounds[index];
+      scene.style.setProperty("--vn-background", `url("assets/${background.file}")`);
+      scene.classList.toggle("vn-dark-dialogue", background.dark === true);
+    };
+    const next = scene.ownerDocument.createElement("button");
+    next.type = "button";
+    next.className = "vn-next";
+    next.dataset.focusKey = "next-background";
+    next.title = "Next background";
+    next.setAttribute("aria-label", "Next background");
+    const icon = scene.ownerDocument.createElement("span");
+    icon.className = "hd-icon";
+    icon.dataset.icon = "arrow-right";
+    icon.setAttribute("aria-hidden", "true");
+    next.append(icon);
+    next.addEventListener("click", () => {
+      index = (index + 1) % backgrounds.length;
+      show();
+    });
+    show();
+    scene.append(next);
+  }
+
+  globalThis.HDVisualNovel = { initialize };
+}());