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/base64.js

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Base64 for media, captured audio and screenshots. Uint8Array.prototype.toBase64
+// and Uint8Array.fromBase64 (Chrome 143+, Firefox 133+) do in about 0.5 ms per
+// megabyte what the String.fromCodePoint/btoa and atob/Uint8Array.from loops
+// take 40–55 ms for, with identical results; both remain as the fallback.
+
+const CHUNK = 0x8000;
+
+export function encodeBase64(data, { btoa: toAscii = globalThis.btoa } = {}) {
+  const bytes = data instanceof Uint8Array ? data : new Uint8Array(data);
+  if (typeof bytes.toBase64 === "function") {
+    try {
+      return bytes.toBase64();
+    } catch {
+      // Fall through to the portable path.
+    }
+  }
+  let binary = "";
+  for (let offset = 0; offset < bytes.length; offset += CHUNK) {
+    binary += String.fromCharCode.apply(null, bytes.subarray(offset, offset + CHUNK));
+  }
+  return toAscii(binary);
+}
+
+export function decodeBase64(text, { atob: fromAscii = globalThis.atob } = {}) {
+  if (typeof Uint8Array.fromBase64 === "function") {
+    try {
+      return Uint8Array.fromBase64(text);
+    } catch {
+      // Let the portable path produce its own result or error.
+    }
+  }
+  const binary = fromAscii(text);
+  const bytes = new Uint8Array(binary.length);
+  for (let index = 0; index < binary.length; index += 1) {
+    bytes[index] = binary.charCodeAt(index);
+  }
+  return bytes;
+}