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/LaborResource/RelationManagers/DefaultPartsRelationManager.php

@@ -36,11 +36,11 @@ class DefaultPartsRelationManager extends RelationManager
                 ->live()
                 ->afterStateUpdated(function ($state, Set $set) {
                     if ($state && $p = Part::find($state)) {
-                        $set('unit', $p->unit);
+                        if ($p->unit_id) { $set('unit_id', $p->unit_id); } else { $set('unit', $p->unit); }
                     }
                 }),
             Forms\Components\TextInput::make('qty')->label(__('Cantitate'))->numeric()->default(1)->required(),
-            Forms\Components\TextInput::make('unit')->label(__('UM'))->default('buc')->maxLength(16),
+            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')),
         ]);
     }
 
@@ -52,7 +52,7 @@ class DefaultPartsRelationManager extends RelationManager
                 Tables\Columns\TextColumn::make('part.name')->label(__('Piesă'))->wrap(),
                 Tables\Columns\TextColumn::make('part.article')->label(__('Cod'))->placeholder('—'),
                 Tables\Columns\TextColumn::make('qty')->label(__('Cant.'))->alignRight(),
-                Tables\Columns\TextColumn::make('unit')->label(__('UM')),
+                Tables\Columns\TextColumn::make('unit_id')->label(__('UM'))->getStateUsing(fn ($record) => $record->unitLabel()),
             ])
             ->headerActions([
                 Actions\CreateAction::make()