feat(i18n): add multi-language UI support with 5 languages

- Add i18n framework with locale detection and cookie persistence
- Support English, Traditional Chinese (zh-TW), Simplified Chinese (zh-CN), Japanese (ja), Korean (ko)
- Add LanguageSelector component in navigation header
- Create locale JSON files with full translations
- Update all page components (root, user, results, history) with i18n support
- Add client-side translation helpers for dynamic content
- Auto-detect user's preferred language from browser settings
- Bump version to v0.1.3

public/i18n.js

@@ -0,0 +1,143 @@
+// Client-side i18n helper
+(function () {
+  const LOCALE_COOKIE_NAME = "locale";
+  const LOCALE_EXPIRY_DAYS = 365;
+
+  // Get current locale from meta tag or cookie
+  function getCurrentLocale() {
+    const metaLocale = document.querySelector("meta[name='locale']")?.content;
+    if (metaLocale) return metaLocale;
+
+    const cookieLocale = getCookie(LOCALE_COOKIE_NAME);
+    if (cookieLocale) return cookieLocale;
+
+    return "en";
+  }
+
+  // Get cookie value
+  function getCookie(name) {
+    const value = `; ${document.cookie}`;
+    const parts = value.split(`; ${name}=`);
+    if (parts.length === 2) return parts.pop().split(";").shift();
+    return null;
+  }
+
+  // Set cookie value
+  function setCookie(name, value, days) {
+    const expires = new Date();
+    expires.setTime(expires.getTime() + days * 24 * 60 * 60 * 1000);
+    document.cookie = `${name}=${value};expires=${expires.toUTCString()};path=/;SameSite=Strict`;
+  }
+
+  // Get translation from window.__TRANSLATIONS__
+  function t(category, key, params) {
+    const translations = window.__TRANSLATIONS__ || {};
+    let text = translations[category]?.[key];
+
+    if (!text) {
+      console.warn(`Missing translation: ${category}.${key}`);
+      return `${category}.${key}`;
+    }
+
+    // Interpolate params
+    if (params) {
+      text = text.replace(/\{(\w+)\}/g, (match, paramKey) => {
+        return params[paramKey] !== undefined ? String(params[paramKey]) : match;
+      });
+    }
+
+    return text;
+  }
+
+  // Expose t function globally
+  window.t = t;
+  window.getCurrentLocale = getCurrentLocale;
+
+  // Language selector functionality
+  document.addEventListener("DOMContentLoaded", () => {
+    const toggle = document.getElementById("language-toggle");
+    const dropdown = document.getElementById("language-dropdown");
+
+    if (!toggle || !dropdown) return;
+
+    // Toggle dropdown
+    toggle.addEventListener("click", (e) => {
+      e.stopPropagation();
+      const isOpen = !dropdown.classList.contains("hidden");
+
+      if (isOpen) {
+        dropdown.classList.add("hidden");
+        dropdown.classList.remove("flex");
+        toggle.setAttribute("aria-expanded", "false");
+      } else {
+        dropdown.classList.remove("hidden");
+        dropdown.classList.add("flex");
+        toggle.setAttribute("aria-expanded", "true");
+      }
+    });
+
+    // Close dropdown when clicking outside
+    document.addEventListener("click", (e) => {
+      if (!toggle.contains(e.target) && !dropdown.contains(e.target)) {
+        dropdown.classList.add("hidden");
+        dropdown.classList.remove("flex");
+        toggle.setAttribute("aria-expanded", "false");
+      }
+    });
+
+    // Handle language selection
+    const options = document.querySelectorAll(".language-option");
+    options.forEach((option) => {
+      option.addEventListener("click", () => {
+        const locale = option.dataset.locale;
+        const webroot = option.dataset.webroot || "";
+
+        // Save to cookie
+        setCookie(LOCALE_COOKIE_NAME, locale, LOCALE_EXPIRY_DAYS);
+
+        // Reload the page to apply the new locale
+        window.location.reload();
+      });
+    });
+
+    // Keyboard navigation for dropdown
+    toggle.addEventListener("keydown", (e) => {
+      if (e.key === "Enter" || e.key === " ") {
+        e.preventDefault();
+        toggle.click();
+      }
+    });
+
+    dropdown.addEventListener("keydown", (e) => {
+      const items = [...dropdown.querySelectorAll(".language-option")];
+      const currentIndex = items.indexOf(document.activeElement);
+
+      switch (e.key) {
+        case "ArrowDown":
+          e.preventDefault();
+          if (currentIndex < items.length - 1) {
+            items[currentIndex + 1].focus();
+          }
+          break;
+        case "ArrowUp":
+          e.preventDefault();
+          if (currentIndex > 0) {
+            items[currentIndex - 1].focus();
+          }
+          break;
+        case "Escape":
+          dropdown.classList.add("hidden");
+          dropdown.classList.remove("flex");
+          toggle.setAttribute("aria-expanded", "false");
+          toggle.focus();
+          break;
+        case "Enter":
+        case " ":
+          if (document.activeElement.classList.contains("language-option")) {
+            document.activeElement.click();
+          }
+          break;
+      }
+    });
+  });
+})();