SEO: add crawlable language navigation links for Google indexing

- Add sr-only CSS class for screen-reader/crawler-visible elements
- Add hidden <nav> with real <a> links to all 6 language versions in template.html
- Googlebot can now discover /de/, /fr/, /es/, /pt-BR/, /ru/ via HTML links
- No visual changes - existing <select> dropdown remains the user-facing switcher
- Rebuild all www/ output with updated CSS hash

website/www/join.html

@@ -4,8 +4,8 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Join Room | KoalaSync</title>
-    <link rel="preload" href="style.e78fc9fc.min.css" as="style">
-    <link rel="stylesheet" href="style.e78fc9fc.min.css">
+    <link rel="preload" href="style.adcf0bc0.min.css" as="style">
+    <link rel="stylesheet" href="style.adcf0bc0.min.css">
     <link rel="icon" type="image/webp" href="assets/NewLogoIcon_64.webp">
     <meta name="robots" content="noindex">