feat(conversion): add SVG to PDF conversion functionality (#5431)

# Description of Changes


This pull request introduces a new SVG to PDF conversion feature,
including both backend and frontend changes. The backend adds a secure,
vector-preserving SVG-to-PDF conversion endpoint with comprehensive SVG
sanitization to prevent XSS and SSRF attacks. The frontend is updated to
route SVG-to-PDF conversions through this new endpoint and to
distinguish SVG from other image formats. Additionally, a new dependency
is added for PDF rendering.

**Backend: SVG to PDF Conversion and Security**

* Adds a new API endpoint and controller (`ConvertSvgToPDF`) for
converting SVG files to PDF, using Batik and PDFBox with vector graphics
preservation and robust error handling.
* Implements SVG sanitization (`SvgSanitizer`) to remove scripts, event
handlers, and dangerous URLs, protecting against XSS and SSRF attacks.
* Introduces a utility (`SvgToPdf`) for rendering SVG to PDF with
timeout protection against resource exhaustion attacks.
* Defines a new request model (`SvgToPdfRequest`) for SVG to PDF
conversion requests.
* Adds the `pdfbox-graphics2d` dependency for vector graphics PDF
rendering.

**Frontend: Routing and Format Handling**

* Updates conversion endpoint constants to add `svg-pdf` and maps SVG
files to use the new `svg-to-pdf` route instead of the generic
image-to-PDF route.
* Removes SVG from the generic image format list and introduces a
dedicated check for SVG format (`isSvgFormat`).
<img width="1133" height="995" alt="image"
src="https://github.com/user-attachments/assets/dec8cf27-ccb9-490d-af76-bff69feb0423"
/>

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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

- [X] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [X] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [X] 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)
- [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](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)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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)

- [X] 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.

---------

Signed-off-by: Balázs Szücs <[email protected]>

frontend/src/core/hooks/tools/convert/useConvertOperation.ts

@@ -15,6 +15,8 @@ export const shouldProcessFilesSeparately = (
     // Image to PDF with combineImages = false
     ((isImageFormat(parameters.fromExtension) || parameters.fromExtension === 'image') &&
      parameters.toExtension === 'pdf' && !parameters.imageOptions.combineImages) ||
+    // SVG to PDF with combineIntoSinglePdf = false
+    (parameters.fromExtension === 'svg' && parameters.toExtension === 'pdf' && !parameters.imageOptions.combineImages) ||
     // PDF to image conversions (each PDF should generate its own image file)
     (parameters.fromExtension === 'pdf' && isImageFormat(parameters.toExtension)) ||
     // PDF to PDF/A conversions (each PDF should be processed separately)
@@ -66,6 +68,8 @@ export const buildConvertFormData = (parameters: ConvertParameters, selectedFile
     formData.append("fitOption", imageOptions.fitOption);
     formData.append("colorType", imageOptions.colorType);
     formData.append("autoRotate", imageOptions.autoRotate.toString());
+  } else if (fromExtension === 'svg' && toExtension === 'pdf') {
+    formData.append("combineIntoSinglePdf", imageOptions.combineImages.toString());
   } else if ((fromExtension === 'html' || fromExtension === 'zip') && toExtension === 'pdf') {
     formData.append("zoom", htmlOptions.zoomLevel.toString());
   } else if ((fromExtension === 'eml' || fromExtension === 'msg') && toExtension === 'pdf') {