Add Achitat + Rest de achitat display fields in payment section

Shows the financial breakdown inline in the Plată & total section:
- Total (MDL) — bold, computed
- Achitat (MDL) — green, from paidAmount()
- Rest de achitat (MDL) — red if unpaid, green if fully paid

All three fields are readOnly/dehydrated (display only). To register
payments, use the Plăți relation manager tab.
Translations added for RO/RU/EN.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lang/en.json

@@ -59,6 +59,9 @@
     "Această acțiune este ireversibilă": "This action is irreversible",
     "Acest email a fost generat automat. Vă rugăm nu răspundeți.": "This email was auto-generated. Please do not reply.",
     "Achitat": "Paid",
+    "Achitat (MDL)": "Paid (MDL)",
+    "Rest de achitat (MDL)": "Balance due (MDL)",
+    "Total (MDL)": "Total (MDL)",
     "Achitat azi": "Paid today",
     "Achitat:": "Paid:",
     "Achiziție piese": "Parts purchase",