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/Filament/Tenant/Resources/WorkOrderResource/RelationManagers/PartsRelationManager.php

@@ -42,7 +42,7 @@ class PartsRelationManager extends RelationManager
                         $set('name', $part->name);
                         $set('article', $part->article);
                         $set('brand', $part->brand);
-                        $set('unit', $part->unit);
+                        if ($part->unit_id) { $set('unit_id', $part->unit_id); } else { $set('unit', $part->unit); }
                         $set('buy_price', $part->buy_price);
                         $set('sell_price', $part->sell_price);
                     }
@@ -52,7 +52,7 @@ class PartsRelationManager extends RelationManager
             Forms\Components\TextInput::make('article')->label(__('Cod articol'))->maxLength(64),
             Forms\Components\TextInput::make('brand')->maxLength(64),
             Forms\Components\TextInput::make('qty')->label(__('Cantitate'))->numeric()->default(1)->required(),
-            Forms\Components\TextInput::make('unit')->label(__('UM'))->maxLength(16)->default('buc'),
+            Forms\Components\Select::make('unit_id')->label(__('UM'))->options(\App\Models\Tenant\Unit::forSelect())->searchable()->default(fn () => \App\Models\Tenant\Unit::where('code','buc')->value('id'))->default('buc'),
             Forms\Components\TextInput::make('buy_price')->label(__('Preț achiziție'))->numeric()->default(0),
             Forms\Components\TextInput::make('sell_price')->label(__('Preț vânzare'))->numeric()->required(),
             Forms\Components\TextInput::make('discount_pct')->label(__('Discount %'))->numeric()->default(0),