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>

app/Models/Tenant/PurchaseItem.php

@@ -12,9 +12,19 @@ class PurchaseItem extends Model
 
     protected $fillable = [
         'company_id', 'purchase_id', 'part_id',
-        'name', 'article', 'qty', 'qty_received', 'unit', 'buy_price', 'total', 'received',
+        'name', 'article', 'qty', 'qty_received', 'unit', 'unit_id', 'buy_price', 'total', 'received',
     ];
 
+    public function unitModel()
+    {
+        return $this->belongsTo(Unit::class, 'unit_id');
+    }
+
+    public function unitLabel(?string $locale = null): string
+    {
+        return Unit::labelFor($this->unit_id, $this->unit, $locale);
+    }
+
     protected $casts = [
         'qty' => 'decimal:2',
         'qty_received' => 'decimal:2',