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/index.html

@@ -401,6 +401,7 @@ button.danger{background:var(--bad);color:#fff;border-color:transparent}
         <div class="card"><h2 data-i18n="sy_display">Display</h2>
           <label class="row"><span class="lb" data-i18n="sy_lang">Language</span>
             <select class="in" id="langSel">
+              <option value="de">Deutsch</option>
               <option value="en">English</option>
               <option value="es">Español</option>
               <option value="fr">Français</option>