feat(conversion): add eBook to PDF via Calibre (EPUB/MOBI/AZW3/FB2/TXT/DOCX) (#4644)

This pull request adds support for converting common eBook formats
(EPUB, MOBI, AZW3, FB2, TXT, DOCX) to PDF using Calibre. It introduces a
new API endpoint and updates the configuration, dependency checks, and
documentation to support this feature. Additionally, it includes related
UI and localization changes.

**New eBook to PDF conversion feature:**

* Added `ConvertEbookToPDFController` with a new
`/api/v1/convert/ebook/pdf` endpoint to handle eBook to PDF conversion
using Calibre, supporting options like embedding fonts, including table
of contents, and page numbers.
* Introduced `ConvertEbookToPdfRequest` model for handling conversion
requests and options.

**Configuration and dependency management:**

* Updated `RuntimePathConfig`, `ApplicationProperties`, and
`ExternalAppDepConfig` to support Calibre's executable path
configuration and dependency checking, ensuring Calibre is available and
correctly integrated.
[[1]](diffhunk://#diff-68c561052c2376c3d494bf11dd821958acd9917b1b2d33a7195ca2d6df7ec517R24)
[[2]](diffhunk://#diff-68c561052c2376c3d494bf11dd821958acd9917b1b2d33a7195ca2d6df7ec517R61)
[[3]](diffhunk://#diff-68c561052c2376c3d494bf11dd821958acd9917b1b2d33a7195ca2d6df7ec517R72-R74)
[[4]](diffhunk://#diff-1c357db0a3e88cf5bedd4a5852415fadad83b8b3b9eb56e67059d8b9d8b10702R359)
[[5]](diffhunk://#diff-8932df49d210349a062949da2ed43ce769b0f107354880a78103664f008f849eR26-R34)
[[6]](diffhunk://#diff-8932df49d210349a062949da2ed43ce769b0f107354880a78103664f008f849eR48)
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[[8]](diffhunk://#diff-8932df49d210349a062949da2ed43ce769b0f107354880a78103664f008f849eR132)
* Registered the new endpoint and tool group in `EndpointConfiguration`,
including logic to enable/disable the feature based on Calibre's
presence.
[[1]](diffhunk://#diff-3cddb66d1cf93eeb8103ccd17cee8ed006e0c0ee006d0ee1cf42d512f177e437R260)
[[2]](diffhunk://#diff-3cddb66d1cf93eeb8103ccd17cee8ed006e0c0ee006d0ee1cf42d512f177e437R440-R442)
[[3]](diffhunk://#diff-3cddb66d1cf93eeb8103ccd17cee8ed006e0c0ee006d0ee1cf42d512f177e437L487-R492)

**Documentation and localization:**

* Updated the `README.md` to mention eBook to PDF conversion support.
* Added UI route and form for eBook to PDF conversion in the web
controller.
* Added English and German localization strings for the new feature,
including descriptions, labels, and error messages.
[[1]](diffhunk://#diff-ee1c6999a33498cfa3abba4a384e73a8b8269856899438de80560c965079a9fdR617-R620)
[[2]](diffhunk://#diff-482633b22866efc985222c4a14efc5b7d2487b59f39b953f038273a39d0362f7R617-R620)
[[3]](diffhunk://#diff-482633b22866efc985222c4a14efc5b7d2487b59f39b953f038273a39d0362f7R1476-R1485)


## 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)
- [ ] 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)

### 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: Anthony Stirling <77850077+Frooodle@users.noreply.github.com>

devGuide/DeveloperGuide.md

@@ -12,6 +12,7 @@ Stirling-PDF is built using:
 - PDFBox
 - LibreOffice
 - qpdf
+- Calibre (`ebook-convert` CLI) for eBook conversions
 - HTML, CSS, JavaScript
 - Docker
 - PDF.js
@@ -54,7 +55,12 @@ Stirling-PDF is built using:
 Stirling-PDF uses Lombok to reduce boilerplate code. Some IDEs, like Eclipse, don't support Lombok out of the box. To set up Lombok in your development environment:
 Visit the [Lombok website](https://projectlombok.org/setup/) for installation instructions specific to your IDE.
 
-5. Add environment variable
+5. Install Calibre CLI (optional but required for eBook conversions)
+   Ensure the `ebook-convert` binary from Calibre is available on your PATH when working on the
+   eBook to PDF feature. The Calibre tool group is automatically disabled when the binary is
+   missing, so having it installed locally allows you to exercise the full workflow.
+
+6. Add environment variable
 For local testing, you should generally be testing the full 'Security' version of Stirling PDF. To do this, you must add the environment flag DISABLE_ADDITIONAL_FEATURES=false to your system and/or IDE build/run step.
 
 ## 4. Project Structure