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/experimental-settings.js

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Settings → Advanced → Experimental features. Rows are built once from the
+// registry in reader-options.js; `render` only reflects the stored flags.
+export function createExperimentalSettings({ document, features, onToggle }) {
+  const list = document.getElementById("experimental-features");
+  const empty = document.getElementById("experimental-empty");
+  const rows = new Map();
+
+  for (const feature of features) {
+    const item = document.createElement("li");
+    const label = document.createElement("label");
+    label.className = "lookup-enable";
+    const name = document.createElement("span");
+    name.textContent = feature.label;
+    const input = document.createElement("input");
+    input.id = `opt-experimental-${feature.id}`;
+    input.type = "checkbox";
+    input.setAttribute("aria-describedby", `${input.id}-help`);
+    input.addEventListener("change", () => onToggle(feature.id, input.checked));
+    label.append(name, input);
+    const hint = document.createElement("p");
+    hint.className = "field-hint";
+    hint.id = `${input.id}-help`;
+    hint.textContent = feature.description;
+    let link = null;
+    if (feature.section) {
+      link = document.createElement("a");
+      link.href = `#${feature.section}`;
+      link.textContent = "Open its settings";
+      hint.append(" ", link);
+    }
+    item.append(label, hint);
+    list.append(item);
+    rows.set(feature.id, { input, link });
+  }
+  empty.hidden = features.length > 0;
+
+  function render(experimental) {
+    for (const [id, { input, link }] of rows) {
+      const enabled = experimental[id] === true;
+      input.checked = enabled;
+      if (link) link.hidden = !enabled;
+    }
+  }
+
+  return { render };
+}