i18n(de): polish German translations & fix typos (Fixes #1532)

[eswarseeram]

This PR polishes the German translation file (src/gui/src/i18n/translations/de.js) by fixing typos, improving wording, and resolving consistency issues. Key fixes include:

• contain: 'Enthalten' (was 'Füllen')
• enter_password_to_confirm_delete_user: corrected typo and wording for account deletion confirmation
• alert_confirm_title: use formal "Sind Sie sicher?" (was informal)
• item_shortcut: 'Verknüpfung' (was 'Abkürzung')
• toolbar.refer: 'Empfehlen' (was 'Verweisen')
• other minor consistency fixes (removed trailing spaces, normalized verbs, etc.)

Please review and let me know if you'd like alternate wording for any entries.