refactor: update renderer.js to use ModalManager for all dialogs

- Import ModalManager and create instances for all 10 dialogs
- Replace classList.add/remove('hidden') with modal.open()/close()
- Remove duplicate backdrop click and escape key handlers (now handled by ModalManager)
- Update print-preview.js to use ModalManager when available
- Add CommonJS export to ModalManager for renderer compatibility

Dialogs updated:
- find-dialog (findModal)
- export-dialog (exportModal)
- print-preview-overlay (printPreviewModal)
- table-generator-dialog (tableModal)
- ascii-art-dialog (asciiModal)
- universal-converter-dialog (converterModal)
- batch-dialog (batchModal)
- pdf-editor-dialog (pdfEditorModal)
- header-footer-dialog (headerFooterModal)
- field-picker-dialog (fieldPickerModal)

Amit Haridas

src/utils/ModalManager.js

@@ -216,3 +216,6 @@ export class ModalManager {
 
 // Export for use in renderer
 window.ModalManager = ModalManager;
+
+// CommonJS export
+module.exports = { ModalManager };