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/Unit.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace App\Models\Tenant;
+
+use App\Models\Concerns\BelongsToTenant;
+use Illuminate\Database\Eloquent\Model;
+
+class Unit extends Model
+{
+    use BelongsToTenant;
+
+    protected $fillable = [
+        'company_id', 'code', 'name_ro', 'name_ru', 'name_en',
+        'is_active', 'sort_order',
+    ];
+
+    protected $casts = [
+        'is_active' => 'bool',
+        'sort_order' => 'int',
+    ];
+
+    /** Returns the localised name (name_ro / name_ru / name_en). Fallback: name_ro, then code. */
+    public function label(?string $locale = null): string
+    {
+        $locale = $locale ?: app()->getLocale();
+        $col = 'name_' . $locale;
+        return $this->{$col} ?: ($this->name_ro ?: $this->code);
+    }
+
+    /** Options array keyed by id for Filament Select. */
+    public static function forSelect(?string $locale = null): array
+    {
+        return static::query()
+            ->where('is_active', true)
+            ->orderBy('sort_order')
+            ->orderBy('code')
+            ->get()
+            ->mapWithKeys(fn (Unit $u) => [$u->id => $u->label($locale)])
+            ->all();
+    }
+
+    /** Look up label from either a Unit id or a legacy code string. */
+    public static function labelFor(?int $unitId, ?string $codeFallback = null, ?string $locale = null): string
+    {
+        if ($unitId) {
+            $u = static::find($unitId);
+            if ($u) return $u->label($locale);
+        }
+        if ($codeFallback) {
+            $u = static::where('code', $codeFallback)->first();
+            if ($u) return $u->label($locale);
+            return $codeFallback;
+        }
+        return '';
+    }
+}