i18n: generate zh-Hant from zh-CN

zh-Hant is the zh-CN text in Traditional characters, but contributors
still translated it by hand from `--missing zh-Hant`, and a model asked
for Traditional Chinese writes Taiwan vocabulary (影片, 伺服器, 載入).

`npm run i18n -- --zh-hant` now rewrites zh-Hant.json from zh-CN.json
with OpenCC s2tw (opencc-js 1.4.2, cn -> tw, characters only), then the
fixed list of characters it picks wrongly. The check that `npm test`
runs fails when zh-Hant differs from that conversion, naming the key,
and `--missing zh-Hant` points to the command instead of printing a
template.

The conversion reproduces every one of the 3346 entries on main. 25
keys were out of zh-CN order and move; no value changes.

The converter loads on first use, so a copy of the script without
node_modules (stale-index.test.mjs) still extracts messages.

package-lock.json

@@ -32,6 +32,7 @@
         "@vue/test-utils": "^2.5.1",
         "flag-icons": "7.5.0",
         "happy-dom": "^20.14.6",
+        "opencc-js": "1.4.2",
         "prettier": "3.9.6",
         "puppeteer": "^25.16.0",
         "sharp": "^0.35.4",
@@ -2976,6 +2977,13 @@
         "node": ">= 0.8"
       }
     },
+    "node_modules/opencc-js": {
+      "version": "1.4.2",
+      "resolved": "https://registry.npmjs.org/opencc-js/-/opencc-js-1.4.2.tgz",
+      "integrity": "sha512-EdcspgdROiCCZDM+jZR6Pa670xW+EW2QhctPBQ8gRjg6lrx4IHEl+2s+kt5izk/TkxVRgdWuFgaGkaEdIUfdaw==",
+      "dev": true,
+      "license": "MIT AND Apache-2.0"
+    },
     "node_modules/pako": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/pako/-/pako-2.1.0.tgz",