feat(conversion): refactor EML parser to use Simple Java Mail library and add MSG support (#5427)

# Description of Changes


Note on Simple Java Mail:
- SJM contains Angus/Jakarta Mail in it.
- SJM is a very thin layer on Angus Mail; see here:
https://github.com/bbottema/simple-java-mail
- SJM gives high level methods to more reliably parse in email via Angus
Mail, but also contains lots of other interesting features.
- SJM is Apache 2 licensed

This pull request updates the email processing utilities to add support
for parsing and validating Outlook MSG files, refactors the
`EmlProcessingUtils` utility class to use instance methods and improved
resource management, and enhances the handling and styling of generated
email HTML. The changes also introduce external CSS resource loading
with a fallback mechanism, and update dependencies to support MSG file
parsing.

**MSG file support and validation:**
- Added `simple-java-mail` and `outlook-module` dependencies to enable
EML and MSG file parsing, and updated validation logic to recognize and
accept MSG files by checking their magic bytes.
(`app/common/build.gradle`, `EmlProcessingUtils.java`)
**Refactoring and modernization of `EmlProcessingUtils`:**
- Converted static methods and fields in `EmlProcessingUtils` to
instance methods/fields, improving testability and future extensibility.
(`EmlProcessingUtils.java`)

**Enhanced HTML/CSS styling for email rendering:**
- Updated HTML generation to use consistent formatting and improved
style variable usage, and refactored CSS injection to load from an
external resource (`email-pdf-styles.css`) with a synchronized cache and
a minimal fallback if the resource is missing.
(`EmlProcessingUtils.java`)
**Attachment and content rendering improvements:**
- Improved the formatting of meta-information (e.g., CC, BCC, Date) and
attachment sections in generated email HTML, and ensured more robust
handling of empty or missing content. (`EmlProcessingUtils.java`)

**General code cleanup and logging:**
- Added SLF4J logging for error handling when loading CSS resources, and
cleaned up imports and method signatures for clarity and
maintainability. (`EmlProcessingUtils.java`)


<img width="367" height="991" alt="image"
src="https://github.com/user-attachments/assets/0cfb959c-da92-4cff-9e52-ff4ab7fa806e"
/>


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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/constants/convertConstants.ts

@@ -96,6 +96,7 @@ export const FROM_FORMAT_OPTIONS = [
   { value: 'txt', label: 'TXT', group: 'Text' },
   { value: 'rtf', label: 'RTF', group: 'Text' },
   { value: 'eml', label: 'EML', group: 'Email' },
+  { value: 'msg', label: 'MSG (Outlook)', group: 'Email' },
   { value: 'epub', label: 'EPUB', group: 'eBook' },
   { value: 'mobi', label: 'MOBI', group: 'eBook' },
   { value: 'azw3', label: 'AZW3', group: 'eBook' },
@@ -140,6 +141,7 @@ export const CONVERSION_MATRIX: Record<string, string[]> = {
   'md': ['pdf'],
   'txt': ['pdf'], 'rtf': ['pdf'],
   'eml': ['pdf'],
+  'msg': ['pdf'],
   'cbr': ['pdf'],
   'epub': ['pdf'], 'mobi': ['pdf'], 'azw3': ['pdf'], 'fb2': ['pdf']
 };
@@ -171,6 +173,7 @@ export const EXTENSION_TO_ENDPOINT: Record<string, Record<string, string>> = {
   'txt': { 'pdf': 'file-to-pdf' }, 'rtf': { 'pdf': 'file-to-pdf' },
   'cbr': { 'pdf': 'cbr-to-pdf' },
   'eml': { 'pdf': 'eml-to-pdf' },
+  'msg': { 'pdf': 'eml-to-pdf' }, // MSG uses same endpoint as EML
   'epub': { 'pdf': 'ebook-to-pdf' }, 'mobi': { 'pdf': 'ebook-to-pdf' }, 'azw3': { 'pdf': 'ebook-to-pdf' }, 'fb2': { 'pdf': 'ebook-to-pdf' }
 };
 

frontend/src/core/constants/convertSupportedFornats.ts

@@ -11,7 +11,7 @@ export const CONVERT_SUPPORTED_FORMATS = [
   // StarOffice
   'sda', 'sdc', 'sdd', 'sdw', 'stc', 'std', 'sti', 'stw', 'sxd', 'sxg', 'sxi', 'sxw',
   // Email formats
-  'eml',
+  'eml', 'msg',
   // Ebook formats
   'epub', 'mobi', 'azw3', 'fb2',
   // Archive formats