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/capture-frame-client.js

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+export function createCaptureFrameEncoder({ WorkerClass = globalThis.Worker,
+  createBitmap = globalThis.createImageBitmap } = {}) {
+  const worker = new WorkerClass(new URL("./capture-frame-worker.js", import.meta.url), {
+    type: "module", name: "hachidori-capture-frames",
+  });
+  let pending = null;
+  let closed = false;
+  const close = (error = new Error("Frame capture stopped.")) => {
+    closed = true;
+    worker.terminate();
+    pending?.reject(error);
+    pending = null;
+  };
+  worker.addEventListener("error", event => {
+    close(event.error ?? new Error(event.message || "The frame encoder stopped."));
+  });
+  worker.addEventListener("message", ({ data }) => {
+    const request = pending;
+    pending = null;
+    if (!request) return;
+    if (data.error) request.reject(new Error(data.error));
+    else request.resolve(data.bytes ? new Uint8Array(data.bytes) : null);
+  });
+  return {
+    async encode(source, dimensions) {
+      if (closed) throw new Error("Frame capture stopped.");
+      // Clone raw VideoFrames without copying pixels; the preview fallback
+      // supplies an ImageBitmap. Only one frame is submitted at a time.
+      const frame = typeof source.clone === "function" ? source.clone() : await createBitmap(source);
+      if (closed || pending) {
+        frame.close();
+        throw new Error(closed ? "Frame capture stopped." : "A capture frame is already being encoded.");
+      }
+      return new Promise((resolve, reject) => {
+        pending = { resolve, reject };
+        try { worker.postMessage({ frame, ...dimensions }, [frame]); }
+        catch (error) {
+          pending = null;
+          frame.close();
+          reject(error);
+        }
+      });
+    },
+    close,
+  };
+}