feat: per-row and global discount with margin validation

Parts (wo_parts) already had discount_pct; now also added to wo_works.
Labor catalog gets a cost_price field as the hard minimum sell floor.

- WorkOrderWork: discount_pct + cost_price fields; total calculation
  now includes discount: hours × price_per_hour × (1 − disc/100)
- Labor model: cost_price field (used to pre-fill work rows and validate)
- WorksRelationManager: discount_pct field with live helper text showing
  effective price + margin %; validation blocks save if below buy cost
  or below company min_discount_margin_pct; "Reducere globală" header
  action applies % to all work rows (skips rows that would breach limits)
- PartsRelationManager: same validation on existing discount_pct field;
  helper text (🟢/🟠/🔴) shows effective price + margin in real time;
  "Reducere globală" header action for all parts
- Settings page: new "Reduceri & limite preț" section with
  min_discount_margin_pct — 0 = no limit, >0 = hard block below that %
- LaborResource form: cost_price field

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Models/Tenant/Labor.php

@@ -23,7 +23,7 @@ class Labor extends Model
 
     protected $fillable = [
         'company_id', 'category', 'name_ro', 'name_ru', 'name_en', 'code',
-        'hours', 'pricing_mode', 'fixed_price', 'price', 'is_active', 'notes',
+        'hours', 'pricing_mode', 'fixed_price', 'price', 'cost_price', 'is_active', 'notes',
     ];
 
     /** Localised name — prefers name_{locale}, falls back to name_ro. */
@@ -38,6 +38,7 @@ class Labor extends Model
         'hours' => 'decimal:2',
         'fixed_price' => 'decimal:2',
         'price' => 'decimal:2',
+        'cost_price' => 'decimal:2',
         'is_active' => 'boolean',
     ];