feat: Admin-Dashboard mit Kundenübersicht, System-Rechnung auf Preistabellen-Positionen umgestellt und Undo im Rechnungsgenerator

- Admin-Dashboard: neue Sektion mit Grid aller Kunden (Kundennummer, Firma, Name, Kontakt, Ort, zugehöriger Nutzer)
- Rechnungsgenerator (System-Template): Positionen ausschließlich aus den drei Parametern der Preistabelle statt Beispiel-Leistungen, in Canvas und PDF-Vorschau; Bausteine Netto-/Bruttosumme entfernt
- AdminLayout: gleicher view-container-Rahmen wie MainLayout, Admin-Seiten damit optisch an Nutzer-Seiten angeglichen
- Canvas-Generatoren (Admin und Profil): Undo-Historie mit bis zu 10 Schritten per Button und Strg+Z
- Übersetzungen für alle 10 Sprachen ergänzt

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/src/main/resources/messages_de.properties

@@ -1077,3 +1077,12 @@ invoicegenerator.notification.cleared=Canvas geleert
 invoicegenerator.notification.canvas.error=Canvas-Daten konnten nicht gelesen werden
 invoicegenerator.notification.preview.error=Fehler bei der Vorschau: {0}
 invoicegenerator.pdf.preview.title=PDF-Vorschau
+
+# Admin Dashboard - Kundenübersicht
+admindashboard.section.customers=Kunden ({0})
+admindashboard.customers.column.number=Kundennummer
+admindashboard.customers.column.owner=Nutzer
+admindashboard.customers.empty=Keine Kunden vorhanden
+invoicegenerator.positions.header=Positionen (Preistabelle)
+invoicegenerator.positions.list=Positionen auflisten
+button.undo=Rückgängig

backend/src/main/resources/messages_ee.properties

@@ -907,3 +907,12 @@ invoicegenerator.notification.cleared=Lõuend tühjendatud
 invoicegenerator.notification.canvas.error=Lõuendi andmeid ei õnnestunud lugeda
 invoicegenerator.notification.preview.error=Eelvaate viga: {0}
 invoicegenerator.pdf.preview.title=PDF-i eelvaade
+
+# Admin Dashboard - Klientide ülevaade
+admindashboard.section.customers=Kliendid ({0})
+admindashboard.customers.column.number=Kliendinumber
+admindashboard.customers.column.owner=Kasutaja
+admindashboard.customers.empty=Kliente pole
+invoicegenerator.positions.header=Positsioonid (hinnatabel)
+invoicegenerator.positions.list=Positsioonide loend
+button.undo=Võta tagasi

backend/src/main/resources/messages_en.properties

@@ -1077,3 +1077,12 @@ invoicegenerator.notification.cleared=Canvas cleared
 invoicegenerator.notification.canvas.error=Could not read canvas data
 invoicegenerator.notification.preview.error=Preview error: {0}
 invoicegenerator.pdf.preview.title=PDF preview
+
+# Admin Dashboard - Customer overview
+admindashboard.section.customers=Customers ({0})
+admindashboard.customers.column.number=Customer number
+admindashboard.customers.column.owner=User
+admindashboard.customers.empty=No customers available
+invoicegenerator.positions.header=Positions (price table)
+invoicegenerator.positions.list=List positions
+button.undo=Undo

backend/src/main/resources/messages_es.properties

@@ -1008,3 +1008,12 @@ invoicegenerator.notification.cleared=Lienzo vaciado
 invoicegenerator.notification.canvas.error=No se pudieron leer los datos del lienzo
 invoicegenerator.notification.preview.error=Error de vista previa: {0}
 invoicegenerator.pdf.preview.title=Vista previa del PDF
+
+# Admin Dashboard - Resumen de clientes
+admindashboard.section.customers=Clientes ({0})
+admindashboard.customers.column.number=Número de cliente
+admindashboard.customers.column.owner=Usuario
+admindashboard.customers.empty=No hay clientes
+invoicegenerator.positions.header=Posiciones (tabla de precios)
+invoicegenerator.positions.list=Listar posiciones
+button.undo=Deshacer

backend/src/main/resources/messages_fr.properties

@@ -1008,3 +1008,12 @@ invoicegenerator.notification.cleared=Canevas vidé
 invoicegenerator.notification.canvas.error=Impossible de lire les données du canevas
 invoicegenerator.notification.preview.error=Erreur d''aperçu : {0}
 invoicegenerator.pdf.preview.title=Aperçu PDF
+
+# Admin Dashboard - Aperçu des clients
+admindashboard.section.customers=Clients ({0})
+admindashboard.customers.column.number=Numéro de client
+admindashboard.customers.column.owner=Utilisateur
+admindashboard.customers.empty=Aucun client
+invoicegenerator.positions.header=Positions (grille tarifaire)
+invoicegenerator.positions.list=Lister les positions
+button.undo=Annuler

backend/src/main/resources/messages_lt.properties

@@ -1010,3 +1010,12 @@ invoicegenerator.notification.cleared=Drobė išvalyta
 invoicegenerator.notification.canvas.error=Nepavyko nuskaityti drobės duomenų
 invoicegenerator.notification.preview.error=Peržiūros klaida: {0}
 invoicegenerator.pdf.preview.title=PDF peržiūra
+
+# Admin Dashboard - Klientų apžvalga
+admindashboard.section.customers=Klientai ({0})
+admindashboard.customers.column.number=Kliento numeris
+admindashboard.customers.column.owner=Naudotojas
+admindashboard.customers.empty=Klientų nėra
+invoicegenerator.positions.header=Pozicijos (kainų lentelė)
+invoicegenerator.positions.list=Pozicijų sąrašas
+button.undo=Anuliuoti

backend/src/main/resources/messages_lv.properties

@@ -1008,3 +1008,12 @@ invoicegenerator.notification.cleared=Audekls notīrīts
 invoicegenerator.notification.canvas.error=Neizdevās nolasīt audekla datus
 invoicegenerator.notification.preview.error=Priekšskatījuma kļūda: {0}
 invoicegenerator.pdf.preview.title=PDF priekšskatījums
+
+# Admin Dashboard - Klientu pārskats
+admindashboard.section.customers=Klienti ({0})
+admindashboard.customers.column.number=Klienta numurs
+admindashboard.customers.column.owner=Lietotājs
+admindashboard.customers.empty=Nav klientu
+invoicegenerator.positions.header=Pozīcijas (cenu tabula)
+invoicegenerator.positions.list=Pozīciju saraksts
+button.undo=Atsaukt

backend/src/main/resources/messages_pl.properties

@@ -1008,3 +1008,12 @@ invoicegenerator.notification.cleared=Obszar roboczy wyczyszczony
 invoicegenerator.notification.canvas.error=Nie można odczytać danych obszaru roboczego
 invoicegenerator.notification.preview.error=Błąd podglądu: {0}
 invoicegenerator.pdf.preview.title=Podgląd PDF
+
+# Admin Dashboard - Przegląd klientów
+admindashboard.section.customers=Klienci ({0})
+admindashboard.customers.column.number=Numer klienta
+admindashboard.customers.column.owner=Użytkownik
+admindashboard.customers.empty=Brak klientów
+invoicegenerator.positions.header=Pozycje (cennik)
+invoicegenerator.positions.list=Lista pozycji
+button.undo=Cofnij

backend/src/main/resources/messages_ru.properties

@@ -1008,3 +1008,12 @@ invoicegenerator.notification.cleared=Холст очищен
 invoicegenerator.notification.canvas.error=Не удалось прочитать данные холста
 invoicegenerator.notification.preview.error=Ошибка предпросмотра: {0}
 invoicegenerator.pdf.preview.title=Предпросмотр PDF
+
+# Admin Dashboard - Обзор клиентов
+admindashboard.section.customers=Клиенты ({0})
+admindashboard.customers.column.number=Номер клиента
+admindashboard.customers.column.owner=Пользователь
+admindashboard.customers.empty=Клиенты отсутствуют
+invoicegenerator.positions.header=Позиции (прайс-лист)
+invoicegenerator.positions.list=Список позиций
+button.undo=Отменить

backend/src/main/resources/messages_tr.properties

@@ -1008,3 +1008,12 @@ invoicegenerator.notification.cleared=Tuval temizlendi
 invoicegenerator.notification.canvas.error=Tuval verileri okunamadı
 invoicegenerator.notification.preview.error=Önizleme hatası: {0}
 invoicegenerator.pdf.preview.title=PDF önizleme
+
+# Admin Dashboard - Müşteri genel bakışı
+admindashboard.section.customers=Müşteriler ({0})
+admindashboard.customers.column.number=Müşteri numarası
+admindashboard.customers.column.owner=Kullanıcı
+admindashboard.customers.empty=Müşteri yok
+invoicegenerator.positions.header=Kalemler (fiyat tablosu)
+invoicegenerator.positions.list=Kalemleri listele
+button.undo=Geri al