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-worker.js

@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+import { MAX_FRAME_BYTES } from "./capture-buffer.js";
+
+let canvas = null;
+let context = null;
+
+async function encodeFrame({ frame, width, height }) {
+  try {
+    if (!canvas) {
+      canvas = new OffscreenCanvas(width, height);
+      context = canvas.getContext("2d", { alpha: false });
+      if (!context) throw new Error("Could not create the capture frame canvas.");
+    }
+    // Keep the initial output size as the captured window changes shape.
+    const sourceWidth = frame.displayWidth || frame.width;
+    const sourceHeight = frame.displayHeight || frame.height;
+    const scale = Math.min(1, canvas.width / sourceWidth, canvas.height / sourceHeight);
+    const drawWidth = sourceWidth * scale;
+    const drawHeight = sourceHeight * scale;
+    context.fillStyle = "#000";
+    context.fillRect(0, 0, canvas.width, canvas.height);
+    context.drawImage(frame, (canvas.width - drawWidth) / 2,
+      (canvas.height - drawHeight) / 2, drawWidth, drawHeight);
+  } finally {
+    frame.close();
+  }
+  // Chrome uses idle tasks for main-thread JPEG encoding, delaying hidden
+  // capture documents by about a second. Worker encoding runs directly.
+  let blob = await canvas.convertToBlob({ type: "image/jpeg", quality: 0.72 });
+  if (blob.size > MAX_FRAME_BYTES) {
+    blob = await canvas.convertToBlob({ type: "image/jpeg", quality: 0.5 });
+  }
+  return blob.size > MAX_FRAME_BYTES ? null : blob.arrayBuffer();
+}
+
+self.addEventListener("message", async ({ data }) => {
+  try {
+    const bytes = await encodeFrame(data);
+    self.postMessage({ bytes }, bytes ? [bytes] : []);
+  } catch (error) {
+    self.postMessage({ error: error.message || String(error) });
+  }
+});