Fix stock import UI bugs: dead code, fragile data attr, double-click, silent duplicates

- Remove unused stockMappingsCache variable (dead code after selectStockMappingRow removal)
- Move data-description from SVG to button element for reliable access
- Add disabled guard on bulk add/ignore buttons to prevent duplicate requests
- Return explicit error in UpsertIgnoreRule when rule already exists

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

internal/services/stock_import.go

@@ -496,11 +496,18 @@ func (s *StockImportService) UpsertIgnoreRule(target, matchType, pattern string)
 	if target == "" || matchType == "" || pattern == "" {
 		return fmt.Errorf("target, match_type and pattern are required")
 	}
-	return s.db.Clauses(clause.OnConflict{DoNothing: true}).Create(&models.StockIgnoreRule{
+	res := s.db.Clauses(clause.OnConflict{DoNothing: true}).Create(&models.StockIgnoreRule{
 		Target:    target,
 		MatchType: matchType,
 		Pattern:   pattern,
-	}).Error
+	})
+	if res.Error != nil {
+		return res.Error
+	}
+	if res.RowsAffected == 0 {
+		return fmt.Errorf("rule already exists")
+	}
+	return nil
 }
 
 func (s *StockImportService) DeleteIgnoreRule(id uint) (int64, error) {