Add Russian to the language site and the forum placeholderfeature/russian-locale

Russian (ru-RU) is the third locale of the language site: the shell, the
home, overview and getting-started pages, and the page metadata are fully
translated, and the language menu offers it. The documentation sidebar reads
its labels from the page messages instead of a two-way locale test, and the
stored preference is accepted only when it names a supported locale.

The forum placeholder gets the same three languages. Its language control
is now a menu instead of a two-state button, and an unknown stored or
selected value keeps the current language.

The site API lists the new locale. Verified locally: prettier, the frontend
tests, vue-tsc and the Vite build, the backend tests, and a browser run of
the home and overview pages in Russian.

3 files changed, 36 insertions, 14 deletions

diff --git a/forum/index.html b/forum/index.html
index 2ad2267..3758b28 100644
--- a/forum/index.html
+++ b/forum/index.html
@@ -18,7 +18,11 @@
         <img src="/visual-xsharp-mark.svg" alt="" />Visual X#
       </a>
       <div class="tools">
-        <button id="language" type="button" aria-label="Switch to German">DE</button>
+        <select id="language" aria-label="Language">
+          <option value="en" lang="en">EN</option>
+          <option value="de" lang="de">DE</option>
+          <option value="ru" lang="ru">RU</option>
+        </select>
         <button id="theme" type="button" aria-label="Switch color theme">☀</button>
       </div>
     </header>
diff --git a/forum/site.css b/forum/site.css
index a4c767e..107faae 100644
--- a/forum/site.css
+++ b/forum/site.css
@@ -55,7 +55,8 @@ header {
   display: flex;
   gap: 8px;
 }
-button {
+button,
+select {
   min-height: 36px;
   padding: 0 12px;
   border: 1px solid var(--line);
@@ -64,10 +65,15 @@ button {
   color: var(--text);
   cursor: pointer;
 }
-button:hover {
+button:hover,
+select:hover {
   border-color: var(--accent);
 }
+select {
+  font: inherit;
+}
 button:focus-visible,
+select:focus-visible,
 a:focus-visible {
   outline: 2px solid var(--accent);
   outline-offset: 3px;
diff --git a/forum/site.js b/forum/site.js
index 030d0ad..9261222 100644
--- a/forum/site.js
+++ b/forum/site.js
@@ -9,8 +9,7 @@ const copy = {
       "We will open the Visual X# forum when the community is ready for it. There are no accounts, posts, or discussion service here today. For now, follow the language and compiler in the public repository.",
     home: "Visual X# home →",
     source: "Source repository ↗",
-    languageButton: "DE",
-    languageLabel: "Switch to German",
+    languageLabel: "Language",
     themeLabel: "Switch color theme",
   },
   de: {
@@ -20,15 +19,25 @@ const copy = {
       "Wir eröffnen das Visual X# Forum, wenn die Community dafür bereit ist. Derzeit gibt es hier keine Konten, Beiträge oder Diskussionen. Folgen Sie bis dahin der Sprache und dem Compiler im öffentlichen Repository.",
     home: "Visual X# Startseite →",
     source: "Quellcode-Repository ↗",
-    languageButton: "EN",
-    languageLabel: "Zu Englisch wechseln",
+    languageLabel: "Sprache",
     themeLabel: "Farbschema wechseln",
   },
+  ru: {
+    eyebrow: "Сообщество / позже",
+    title: "Форум пока не открыт.",
+    message:
+      "Мы откроем форум Visual X#, когда сообщество будет к этому готово. Сейчас здесь нет ни аккаунтов, ни сообщений, ни сервиса обсуждений. Пока следите за языком и компилятором в публичном репозитории.",
+    home: "На главную Visual X# →",
+    source: "Репозиторий с исходным кодом ↗",
+    languageLabel: "Язык",
+    themeLabel: "Сменить цветовую тему",
+  },
 };
 
-const languageButton = document.getElementById("language");
+const languageSelect = document.getElementById("language");
 const themeButton = document.getElementById("theme");
-let language = localStorage.getItem("vxs-forum-language") === "de" ? "de" : "en";
+const storedLanguage = localStorage.getItem("vxs-forum-language");
+let language = Object.hasOwn(copy, storedLanguage ?? "") ? storedLanguage : "en";
 
 function renderLanguage() {
   const selected = copy[language];
@@ -37,14 +46,17 @@ function renderLanguage() {
   for (const key of ["eyebrow", "title", "message", "home", "source"]) {
     document.getElementById(key).textContent = selected[key];
   }
-  languageButton.textContent = selected.languageButton;
-  languageButton.setAttribute("aria-label", selected.languageLabel);
+  languageSelect.value = language;
+  languageSelect.setAttribute("aria-label", selected.languageLabel);
   themeButton.setAttribute("aria-label", selected.themeLabel);
 }
 
-languageButton.addEventListener("click", () => {
-  language = language === "en" ? "de" : "en";
-  localStorage.setItem("vxs-forum-language", language);
+languageSelect.addEventListener("change", () => {
+  // The menu only offers known languages; anything else keeps the current one.
+  if (Object.hasOwn(copy, languageSelect.value)) {
+    language = languageSelect.value;
+    localStorage.setItem("vxs-forum-language", language);
+  }
   renderLanguage();
 });