web: add German (de) translation

Adds a German dictionary to LANG in app.js (all 317 keys, placeholders
unchanged), the language selector entry in index.html and the login
strings in login.html. Uses Swiss spelling (ss instead of ß), which
reads correctly for all German speakers.

Checked against a mock of the httpd endpoints: every section renders
with the German strings; no layout changes compared to English.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

html/login.html

@@ -81,6 +81,7 @@ button:hover{filter:brightness(1.1)}
 <script>
 (function(){
   var L={
+    de:["Switch-Anmeldung","RTLPlayground-Verwaltungsoberfläche","Passwort","Anmelden","Falsches Passwort, bitte erneut versuchen."],
     en:["Switch Login","RTLPlayground management interface","Password","Sign in","Wrong password, try again."],
     ja:["スイッチ ログイン","RTLPlayground 管理インターフェース","パスワード","ログイン","パスワードが違います。もう一度お試しください。"],
     zh:["交换机登录","RTLPlayground 管理界面","密码","登录","密码错误，请重试。"],