refactor(i18n): move translations to JSON files under app/locales, dynamic language switch

app/templates/base.html

@@ -19,8 +19,11 @@
             {% endif %}
             <a href="/api/docs">{{ 'nav.api_docs'|t(user.language) }}</a>
             <form method="post" action="/settings/language" class="inline" style="display:inline-flex;align-items:center;gap:0">
-                <input type="hidden" name="language" value="{{ 'en' if user.language == 'zh' else 'zh' }}">
-                <button type="submit" class="nav-lang-btn">{{ 'EN' if user.language == 'zh' else '中' }}</button>
+                <select name="language" class="nav-lang-select" onchange="this.form.submit()">
+                    {% for code, name in available_languages.items() %}
+                    <option value="{{ code }}" {% if user.language == code %}selected{% endif %}>{{ name }}</option>
+                    {% endfor %}
+                </select>
             </form>
             <a href="/logout" class="nav-logout">{{ 'nav.logout'|t(user.language) }}</a>
             {% endif %}