feat: localize interview questions (ko/en), send X-Lang header

internal/webapp/static/app.js

@@ -89,6 +89,7 @@ els.sessionForm.addEventListener("submit", async (event) => {
       .map((item) => item.trim())
       .filter(Boolean),
     interview_timeline: value("#timeline"),
+    lang: localStorage.getItem("tutor_lang") || document.documentElement.lang || "ko",
   };
 
   try {
@@ -216,7 +217,7 @@ function renderSession() {
       "aria-pressed",
       String(state.selectedQuestion?.id === question.id)
     );
-    button.innerHTML = `<span class="question-id">${escapeHTML(question.id)}</span>${escapeHTML(question.prompt)}`;
+    button.innerHTML = `<span class="question-id">${escapeHTML(question.id)}</span>${escapeHTML(tq(question.id) || question.prompt)}`;
     button.addEventListener("click", () => {
       state.selectedQuestion = question;
       els.answerText.value = "";
@@ -465,7 +466,8 @@ els.logoutButton.addEventListener("click", () => {
 
 async function request(url, options = {}) {
   const token = localStorage.getItem("tutor_token");
-  const headers = { "Content-Type": "application/json" };
+  const lang = localStorage.getItem("tutor_lang") || document.documentElement.lang || "ko";
+  const headers = { "Content-Type": "application/json", "X-Lang": lang };
   if (token) headers["Authorization"] = `Bearer ${token}`;
   const response = await fetch(url, { headers, ...options });
   const body = await response.json();

internal/webapp/static/i18n.js

@@ -181,6 +181,33 @@ window.t = function (key, ...args) {
   return typeof text === "function" ? text(...args) : text;
 }
 
+var questionTexts = {
+  ko: {
+    "backend-http-idempotency":
+      "HTTP 메서드가 멱등성을 가지려면 어떤 조건이 필요하며, 재시도 시 왜 중요한가요?",
+    "backend-db-index-tradeoff":
+      "데이터베이스 인덱스를 추가하면 API가 어떻게 개선되며, 어떤 트레이드오프가 발생할 수 있나요?",
+    "backend-cache-invalidation":
+      "API 응답을 캐싱할지 어떻게 결정하며, 오래된 데이터는 어떻게 처리하나요?",
+  },
+  en: {
+    "backend-http-idempotency":
+      "What makes an HTTP method idempotent, and why does that matter for retries?",
+    "backend-db-index-tradeoff":
+      "When would adding a database index improve an API, and what tradeoffs can it introduce?",
+    "backend-cache-invalidation":
+      "How would you decide whether to cache an API response, and how would you handle stale data?",
+  },
+};
+
+window.tq = function (id) {
+  const lang =
+    localStorage.getItem("tutor_lang") ||
+    document.documentElement.lang ||
+    "ko";
+  return questionTexts[lang]?.[id] ?? questionTexts["en"]?.[id] ?? "";
+};
+
 window.updateStaticText = function () {
   const lang =
     localStorage.getItem("tutor_lang") ||