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.

1 files changed, 4 insertions, 2 deletions

diff --git a/frontend/src/preferences.ts b/frontend/src/preferences.ts
index 878bdce..e5078bc 100644
--- a/frontend/src/preferences.ts
+++ b/frontend/src/preferences.ts
@@ -3,11 +3,13 @@
 
 import { ref, watch } from "vue";
 
-export type Locale = "en-US" | "de-DE";
+export const supportedLocales = ["en-US", "de-DE", "ru-RU"] as const;
+export type Locale = (typeof supportedLocales)[number];
 export type Theme = "dark" | "light";
 
 function storedLocale(): Locale {
-  return localStorage.getItem("vxs-locale") === "de-DE" ? "de-DE" : "en-US";
+  const stored = localStorage.getItem("vxs-locale");
+  return supportedLocales.find((supported) => supported === stored) ?? "en-US";
 }
 
 function storedTheme(): Theme {