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/dictionary-import-archive.js

@@ -0,0 +1,84 @@
+/*
+ * Reads only Yomitan index.json for an interactive import decision. The engine
+ * still imports and validates the complete archive before anything is published.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+import { BlobReader, Writer, ZipReader } from "./vendor/zip.js";
+import { dictionaryArchiveIdentity } from "./dictionary-import.js";
+
+export const MAX_PREFLIGHT_INDEX_BYTES = 1024 * 1024;
+
+export class BoundedIndexWriter extends Writer {
+  constructor(limit) {
+    super();
+    this.limit = limit;
+    this.length = 0;
+    this.parts = [];
+  }
+
+  writeUint8Array(bytes) {
+    if (this.length + bytes.byteLength > this.limit) {
+      throw new Error(`The selected archive's index.json exceeds ${this.limit} bytes.`);
+    }
+    this.parts.push(bytes.slice());
+    this.length += bytes.byteLength;
+  }
+
+  getData() {
+    const data = new Uint8Array(this.length);
+    let offset = 0;
+    for (const part of this.parts) {
+      data.set(part, offset);
+      offset += part.byteLength;
+    }
+    return new TextDecoder().decode(data);
+  }
+}
+
+function assertRegularIndex(entry) {
+  const kind = entry.unixMode === undefined ? 0 : entry.unixMode & 0o170000;
+  if (entry.directory || entry.symlink || (kind !== 0 && kind !== 0o100000)) {
+    throw new Error("The selected archive's index.json is not a regular file.");
+  }
+  if (entry.encrypted) {
+    throw new Error("The selected archive's index.json is encrypted.");
+  }
+}
+
+export async function readDictionaryArchiveIdentity(blob) {
+  const reader = new ZipReader(new BlobReader(blob), {
+    useWebWorkers: false,
+    checkAmbiguity: true,
+  });
+  try {
+    const entries = await reader.getEntries();
+    const indexes = entries.filter(entry => entry.filename === "index.json");
+    if (indexes.length !== 1) {
+      throw new Error(indexes.length === 0
+        ? "The selected archive has no index.json."
+        : "The selected archive has more than one index.json.");
+    }
+    const entry = indexes[0];
+    assertRegularIndex(entry);
+    if (entry.uncompressedSize > MAX_PREFLIGHT_INDEX_BYTES) {
+      throw new Error(`The selected archive's index.json exceeds ${MAX_PREFLIGHT_INDEX_BYTES} bytes.`);
+    }
+    const data = await entry.getData(new BoundedIndexWriter(MAX_PREFLIGHT_INDEX_BYTES), {
+      useWebWorkers: false,
+      checkSignature: true,
+      checkOverlappingEntry: true,
+      checkAmbiguity: true,
+    });
+    let index;
+    try {
+      index = JSON.parse(data);
+    } catch (error) {
+      throw new Error(`The selected archive's index.json is malformed: ${error.message || String(error)}`);
+    }
+    return dictionaryArchiveIdentity(index);
+  } finally {
+    await reader.close();
+  }
+}