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/browser-api.js

@@ -0,0 +1,44 @@
+// Browser API and extension-origin facts shared by module contexts.
+// Content scripts intentionally keep their callback-style `chrome` calls.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+export function selectExtensionApi(scope = globalThis) {
+  return scope.browser ?? scope.chrome ?? null;
+}
+
+export const extensionApi = selectExtensionApi();
+
+export function extensionProtocol(api = extensionApi) {
+  const url = api?.runtime?.getURL?.("");
+  if (typeof url !== "string" || url === "") return "";
+  try {
+    return new URL(url).protocol;
+  } catch {
+    return "";
+  }
+}
+
+export function browserKind(api = extensionApi) {
+  return extensionProtocol(api) === "moz-extension:" ? "firefox" : "chrome";
+}
+
+export const BROWSER_KIND = browserKind();
+export const IS_FIREFOX = BROWSER_KIND === "firefox";
+
+export function extensionDocumentUrl(path, api = extensionApi) {
+  return api?.runtime?.getURL?.(path) ?? path;
+}
+
+export function expectedBackgroundUrl(api = extensionApi) {
+  return extensionDocumentUrl(
+    browserKind(api) === "firefox" ? "firefox-background.html" : "background.js",
+    api,
+  );
+}
+
+export function isExactExtensionSender(sender, path, api = extensionApi, { tab = null } = {}) {
+  if (sender?.id !== api?.runtime?.id || sender?.url !== extensionDocumentUrl(path, api)) return false;
+  if (tab === false && sender.tab !== undefined) return false;
+  if (tab === true && sender.tab === undefined) return false;
+  return true;
+}