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-encoder-worker.js

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+import createAvifEncoderModule from "./vendor/avif-encoder.mjs";
+import { encodeJpegSequence } from "./avif-sequence.js";
+
+let modulePromise;
+
+function encoderModule() {
+  modulePromise ??= createAvifEncoderModule({
+    locateFile: name => new URL(`./vendor/${name}`, import.meta.url).href,
+  });
+  return modulePromise;
+}
+
+// A dedicated worker receives messages only from its owning Worker. Window
+// MessageEvent.origin checks do not form a security boundary in this context.
+self.addEventListener("message", async event => { // NOSONAR -- S2819 applies to Window messaging, not this worker.
+  const request = event.data;
+  if (request?.type !== "encode" || typeof request.id !== "string") return;
+  try {
+    const module = await encoderModule();
+    const data = await encodeJpegSequence(module, request.frames, {
+      endMs: request.endMs,
+      quality: request.videoPreset === "compact" ? 50 : 55,
+      speed: 8,
+      onProgress(completed, total) {
+        self.postMessage({ type: "progress", id: request.id, completed, total,
+          heapBytes: module.HEAPU8.byteLength });
+      },
+    });
+    // WebAssembly memory only grows; this includes any final muxing allocation.
+    self.postMessage({ type: "progress", id: request.id,
+      completed: request.frames.length, total: request.frames.length,
+      heapBytes: module.HEAPU8.byteLength });
+    self.postMessage({ type: "result", id: request.id, ok: true, data: data.buffer }, [data.buffer]);
+  } catch (error) {
+    self.postMessage({ type: "result", id: request.id, ok: false,
+      error: error instanceof Error ? error.message : String(error) });
+  }
+});