chore(i18n): key-parity + placeholder check script, catches untranslated keys

scripts/check-i18n.mjs

@@ -0,0 +1,32 @@
+// i18n key-parity check: every locale file must have exactly en.json's keys,
+// and every translation must use the same {placeholders} as its English
+// source. Runs as part of `npm run lint` — English fallback otherwise hides
+// missing translations silently (they shipped untranslated for months once).
+import { readFileSync, readdirSync } from "node:fs";
+
+const DIR = new URL("../src/lib/i18n/", import.meta.url);
+const en = JSON.parse(readFileSync(new URL("en.json", DIR), "utf8"));
+const placeholders = (s) =>
+  [...s.matchAll(/\{(\w+)\}/g)]
+    .map((m) => m[1])
+    .sort()
+    .join(",");
+
+let failed = false;
+for (const file of readdirSync(DIR).filter((f) => f.endsWith(".json") && f !== "en.json")) {
+  const dict = JSON.parse(readFileSync(new URL(file, DIR), "utf8"));
+  const missing = Object.keys(en).filter((k) => !(k in dict));
+  const extra = Object.keys(dict).filter((k) => !(k in en));
+  const badVars = Object.keys(dict).filter(
+    (k) => k in en && placeholders(dict[k]) !== placeholders(en[k]),
+  );
+  for (const k of missing) console.error(`${file}: missing "${k}"`);
+  for (const k of extra) console.error(`${file}: extra "${k}" (not in en.json)`);
+  for (const k of badVars) console.error(`${file}: "${k}" placeholders differ from en.json`);
+  if (missing.length || extra.length || badVars.length) failed = true;
+}
+if (failed) {
+  console.error("\ni18n parity check FAILED — keep every locale in lockstep with en.json.");
+  process.exit(1);
+}
+console.log("i18n parity OK");