feat(accounting): re-viewable incoming invoices + expense invoiced/paid lifecycle UI

#1 Incoming invoices are re-viewable: extracted a reusable rasterised
DocumentPreview (last page = QR-bill), added a click-to-view ViewModal on
every row, and embedded the preview in the mark-paid dialog.

#2/#3 Expenses ledger:
- invoiced badge (links to the client invoice) + paid toggle (manual,
  independent of invoiced)
- edit until invoiced (ExpenseFormModal now does create + edit; locked
  rows show a Lock chip instead of edit/add-to-invoice)
- 'Add to invoice' action (re-bill via customer picker + markup) and a
  'Mark paid' dialog
- service: Expense gains invoiced/billedInvoiceId/paid/paidAt fields +
  invoiceExpense() and markExpensePaid()

en/de translations added.

frontend/src/i18n/locales/de.json

@@ -3474,6 +3474,7 @@
       "triageTitle": "Dokument kategorisieren",
       "saveCategorize": "Speichern",
       "categorize": "Kategorisieren",
+      "view": "Ansehen",
       "empty": "Noch keine Dokumente — oben eines erfassen.",
       "untitled": "Unbenanntes Dokument",
       "noAmount": "Betrag nicht erfasst",
@@ -3485,6 +3486,7 @@
       "nextPage": "Weiter",
       "pageOf": "Seite {{n}} / {{total}}",
       "status": {
+        "label": "Status",
         "unsorted": "Neu",
         "categorized": "Kategorisiert",
         "declined": "Abgelehnt",
@@ -3545,6 +3547,7 @@
       "whoHint": "z. B. Mitarbeitername oder Geschäft",
       "proof": "Beleg",
       "proofRequired": "Ein Beleg ist erforderlich.",
+      "proofExisting": "Es ist bereits ein Beleg angehängt — neuen hochladen, um ihn zu ersetzen.",
       "viewProof": "Beleg"
     },
     "expenseKind": {
@@ -3582,6 +3585,16 @@
       "unpaidToast": "Als nicht bezahlt markiert.",
       "addExpense": "Aufwand hinzufügen",
       "addTitle": "Aufwand hinzufügen",
+      "editTitle": "Aufwand bearbeiten",
+      "updatedToast": "Aufwand aktualisiert.",
+      "payTitle": "Aufwand als bezahlt markieren",
+      "invoiced": "Verrechnet",
+      "invoicedToast": "Zu einer Kundenrechnung hinzugefügt.",
+      "invoiceTitle": "Zur Kundenrechnung hinzufügen",
+      "invoiceHint": "Erstellt eine verrechenbare Position auf der nächsten geplanten Rechnung des Kunden und sperrt den Aufwand für weitere Änderungen.",
+      "addToInvoice": "Zur Rechnung hinzufügen",
+      "locked": "Gesperrt",
+      "lockedHint": "Gesperrt — dieser Aufwand ist auf einer Kundenrechnung.",
       "description": "Beschreibung",
       "descriptionHint": "z. B. Kilometer, Spesenpauschale, Barbeleg",
       "createdToast": "Aufwand hinzugefügt."

frontend/src/i18n/locales/en.json

@@ -3474,6 +3474,7 @@
       "triageTitle": "Categorize document",
       "saveCategorize": "Save",
       "categorize": "Categorize",
+      "view": "View",
       "empty": "No documents yet — capture one above.",
       "untitled": "Untitled document",
       "noAmount": "amount not entered",
@@ -3485,6 +3486,7 @@
       "nextPage": "Next",
       "pageOf": "Page {{n}} / {{total}}",
       "status": {
+        "label": "Status",
         "unsorted": "New",
         "categorized": "Categorized",
         "declined": "Declined",
@@ -3545,6 +3547,7 @@
       "whoHint": "e.g. coworker name or shop",
       "proof": "Proof",
       "proofRequired": "A proof file is required.",
+      "proofExisting": "A proof file is already attached — upload a new one to replace it.",
       "viewProof": "Proof"
     },
     "expenseKind": {
@@ -3582,6 +3585,16 @@
       "unpaidToast": "Marked as not paid.",
       "addExpense": "Add expense",
       "addTitle": "Add expense",
+      "editTitle": "Edit expense",
+      "updatedToast": "Expense updated.",
+      "payTitle": "Mark expense paid",
+      "invoiced": "Invoiced",
+      "invoicedToast": "Added to a client invoice.",
+      "invoiceTitle": "Add to client invoice",
+      "invoiceHint": "This creates a billable line on the client’s next scheduled invoice and locks the expense from further edits.",
+      "addToInvoice": "Add to invoice",
+      "locked": "Locked",
+      "lockedHint": "Locked — this expense is on a client invoice.",
       "description": "Description",
       "descriptionHint": "e.g. mileage, per-diem, cash receipt",
       "createdToast": "Expense added."