Feature/v2/improve sign (#4627)

# Description of Changes

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Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] 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](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: James Brunton <[email protected]>
Co-authored-by: Claude <[email protected]>

frontend/package-lock.json

@@ -54,6 +54,7 @@
         "react-dom": "^19.1.1",
         "react-i18next": "^15.7.3",
         "react-router-dom": "^7.9.1",
+        "signature_pad": "^5.0.4",
         "tailwindcss": "^4.1.13",
         "web-vitals": "^5.1.0"
       },
@@ -9992,6 +9993,12 @@
       "dev": true,
       "license": "ISC"
     },
+    "node_modules/signature_pad": {
+      "version": "5.1.1",
+      "resolved": "https://registry.npmjs.org/signature_pad/-/signature_pad-5.1.1.tgz",
+      "integrity": "sha512-BT5JJygS5BS0oV+tffPRorIud6q17bM7v/1LdQwd0o6mTqGoI25yY1NjSL99OqkekWltS4uon6p52Y8j1Zqu7g==",
+      "license": "MIT"
+    },
     "node_modules/slash": {
       "version": "5.1.0",
       "resolved": "https://registry.npmjs.org/slash/-/slash-5.1.0.tgz",