Fix competitor price display and pricelist item deduplication

- Render competitor prices in Pricing tab (all three row branches)
- Add footer total accumulation for competitor column
- Deduplicate local_pricelist_items via migration + unique index
- Use ON CONFLICT DO NOTHING in SaveLocalPricelistItems to prevent duplicates on concurrent sync

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

internal/localdb/localdb.go

@@ -1147,20 +1147,20 @@ func (l *LocalDB) CountLocalPricelistItemsWithEmptyCategory(pricelistID uint) (i
 	return count, nil
 }
 
-// SaveLocalPricelistItems saves pricelist items to local SQLite
+// SaveLocalPricelistItems saves pricelist items to local SQLite.
+// Duplicate (pricelist_id, lot_name) rows are silently ignored.
 func (l *LocalDB) SaveLocalPricelistItems(items []LocalPricelistItem) error {
 	if len(items) == 0 {
 		return nil
 	}
 
-	// Batch insert
 	batchSize := 500
 	for i := 0; i < len(items); i += batchSize {
 		end := i + batchSize
 		if end > len(items) {
 			end = len(items)
 		}
-		if err := l.db.CreateInBatches(items[i:end], batchSize).Error; err != nil {
+		if err := l.db.Clauses(clause.OnConflict{DoNothing: true}).CreateInBatches(items[i:end], batchSize).Error; err != nil {
 			return err
 		}
 	}