fix(accounting): PDF pager always shown, click categorized→pay, drop duplicate Paid chip

#1 DocumentPreview renders the page pager for every PDF (disabled at the ends),
   not only multi-page ones — so the control is visible on single-page invoices.
#2 Clicking a categorized (unpaid) invoice opens the Mark-paid dialog; new →
   categorize, paid/declined/duplicate → view.
#3 A paid row no longer shows two 'Paid' chips — the front badge is the status,
   and the right action becomes a quiet 'Mark unpaid' (revert).

frontend/src/i18n/locales/de.json

@@ -3563,6 +3563,7 @@
       "payTitle": "Lieferant als bezahlt markieren",
       "outstanding": "Offen",
       "markPaid": "Als bezahlt markieren",
+      "markUnpaid": "Als unbezahlt markieren",
       "confirmPaid": "Als bezahlt markieren",
       "paid": "Bezahlt",
       "paidToast": "Als bezahlt markiert.",

frontend/src/i18n/locales/en.json

@@ -3563,6 +3563,7 @@
       "payTitle": "Mark supplier paid",
       "outstanding": "Outstanding",
       "markPaid": "Mark paid",
+      "markUnpaid": "Mark unpaid",
       "confirmPaid": "Mark paid",
       "paid": "Paid",
       "paidToast": "Marked as paid.",