feat(multitool): Add duplicate page action with undo/redo and harden command stack #4322

[Ludy87]

Description of Changes

• Introduced Duplicate Page action in the MultiTool UI:
  • New DuplicatePageCommand with full undo/redo support
  • Button added next to Delete, with tooltip text wired via i18n (multiTool.duplicate)
  • PdfContainer.duplicatePage() clones page image, rotation, and adapter state; inserts after source; refreshes numbering
• Hooked duplication into the actions pipeline:
  • PdfActionsManager now receives duplicatePage via setActions(...)
  • Added duplicatePageButtonCallback and control markup creation
• Refactored command architecture for robust undo/redo:
  • Added CommandWithAnchors for capturing positions and deterministic reinsertion (used by add/duplicate/page-break)
  • Modernized AddFilesCommand, PageBreakCommand to use anchors, avoid DOM state on nodes, and handle arrays consistently
  • Improved RemoveSelectedCommand, DeletePageCommand, MovePageCommand, Rotate* and Split* commands:
    • Safer null checks for filename/export controls
    • Clearer semantics and documentation
    • Consistent redo mirroring execute
• Minor UI/UX fix:
  • Corrected tooltip wiring for the Add File button (insertFileButton.setAttribute('title', ...))
• Internationalization:
  • Added multiTool.duplicate=Duplicate to messages_en_GB.properties
  • Exposed translations.duplicate in multi-tool.html

Why: Adds a commonly requested workflow improvement (duplicate a single page quickly) and makes the command stack more resilient (especially redo after complex DOM mutations), improving reliability and maintainability.

before:

after:

Closes #4322

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Checklist

General

• [x] I have read the Contribution Guidelines
• [x] I have read the Stirling-PDF Developer Guide (if applicable)
• [ ] I have read the How to add new languages to Stirling-PDF (if applicable)
• [x] I have performed a self-review of my own code
• [x] My changes generate no new warnings

Documentation

• [ ] I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• [ ] I have read the section Add New Translation Tags (for new translation tags only)

UI Changes (if applicable)

• [x] Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• [ ] I have tested my changes locally. Refer to the Testing Guide for more details.

[stirlingbot[bot]]

🚀 Translation Verification Summary

🔄 Reference Branch: pr-branch-messages_en_GB.properties

📃 File Check: messages_en_GB.properties

1. Test Status: ✅ Passed
2. Test Status: ✅ Passed
3. Test Status: ✅ Passed

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✅ Overall Check Status: Success

Thanks @Ludy87 for your help in keeping the translations up to date.

[Ludy87]

/prdeploy

[Ludy87]

/prdeploy

[Frooodle]

/prdeploy