feat(units): add Unit-of-measure nomenclator (per-tenant, 3 langs)

- Migration: create `units` table + nullable unit_id FK on
  parts / wo_parts / purchase_items / labor_parts. Seeds ~11 standard
  units (buc/set/l/ml/kg/g/m/cm/m²/oră/pack) for every existing
  tenant and backfills unit_id by matching the legacy string `unit`
  column. Keeps `unit` string as fallback so old code paths keep
  rendering.
- Unit model: label(locale), forSelect(locale), labelFor(id, code)
  helpers. All 4 owner models get unitModel() relation + unitLabel()
  accessor.
- UnitResource under Depozit group with Filament UI: code, sort,
  is_active + separate name_ro/name_ru/name_en fields.
- Filament forms/tables updated: Part / PurchaseItem / LaborPart /
  WorkOrderPart now use Select('unit_id')->options(Unit::forSelect())
  for input and TextColumn->getStateUsing(unitLabel()) for display.
  Selecting a part auto-fills unit_id when the source has one.
- +10 translations for the new resource + defaults; nav.label
  'Unități de măsură' added to all 3 lang files.

All 306 tests pass.

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

lang/en/nav.php

@@ -38,6 +38,7 @@ return [
         'Companii' => 'Companies',
         'Depozit' => 'Warehouse',
         'Depozite' => 'Warehouses',
+        'Unități de măsură' => 'Units of measure',
         'Facturi & abonamente' => 'Invoices & subscriptions',
         'Finanțe (consolidat)' => 'Finance (consolidated)',
         'Fișe lucru' => 'Work orders',