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, 8 insertions, 2 deletions

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;