i18n: add missing 'piesă' key + fix 'piese' back to nominative

'piesă' (lowercase, Part::getModelLabel()) was identity-fallback so
Filament rendered 'Редактирование Piesă' instead of 'Редактирование
запчасть'. Also 'piese' was translated to genitive ('запчастей') from
an earlier pass where it was used as a count suffix — reverted to
nominative since it's the model label proper.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lang/en.json

@@ -2048,7 +2048,7 @@
   "null = fără trial": "null = fără trial",
   "pentru a alege": "to select",
   "piese": "parts",
-  "piesă": "piesă",
+  "piesă": "part",
   "pk_test_...": "pk_test_...",
   "plan": "plan",
   "planuri": "planuri",

lang/ru.json

@@ -2047,8 +2047,8 @@
   "normă": "норма",
   "null = fără trial": "null = fără trial",
   "pentru a alege": "чтобы выбрать",
-  "piese": "запчастей",
+  "piese": "запчасти",
-  "piesă": "piesă",
+  "piesă": "запчасть",
   "pk_test_...": "pk_test_...",
   "plan": "plan",
   "planuri": "planuri",