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

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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/public/locales/en-GB/translation.toml

@@ -1295,6 +1295,10 @@ cbzDpi = "DPI for image rendering"
 cbrOptions = "CBR Options"
 cbrOutputOptions = "PDF to CBR Options"
 cbrDpi = "DPI for image rendering"
+svgPdfOptions = "SVG to PDF Options"
+combineSvgs = "Combine SVGs into single PDF"
+combineSvgsDescription = "Combine all SVG files into one PDF with multiple pages, or create separate PDFs for each SVG"
+svgVectorNote = "SVG files are rendered as vector graphics for crisp output at any resolution. Dimensions from the SVG determine the PDF page size."
 
 [convert.ebookOptions]
 ebookOptions = "eBook to PDF Options"
@@ -1396,6 +1400,7 @@ applySignatures = "Apply Images"
 name = "Image"
 placeholder = "Upload an image"
 label = "Image file"
+hint = "Supports PNG, JPEG, GIF, BMP, TIFF, and SVG files. SVG files will be converted to PNG for embedding."
 
 [addImage.steps]
 configure = "Configure Image"