refactor: move modules under app/ directory and update file paths (#3938)

# Description of Changes

- **What was changed:**  
- Renamed top-level directories: `stirling-pdf` → `app/core`, `common` →
`app/common`, `proprietary` → `app/proprietary`.
- Updated all path references in `.gitattributes`, GitHub workflows
(`.github/workflows/*`), scripts (`.github/scripts/*`), `.gitignore`,
Dockerfiles, license files, and template settings to reflect the new
structure.
- Added a new CI job `check-generateOpenApiDocs` to generate and upload
OpenAPI documentation.
- Removed redundant `@Autowired` annotations from `TempFileShutdownHook`
and `UnlockPDFFormsController`.
- Minor formatting and comment adjustments in YAML templates and
resource files.

- **Why the change was made:**  
- To introduce a clear `app/` directory hierarchy for core, common, and
proprietary modules, improving organization and maintainability.
- To ensure continuous integration and Docker builds continue to work
seamlessly with the reorganized structure.
- To automate OpenAPI documentation generation as part of the CI
pipeline.

---

## 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.

devGuide/HowToAddNewLanguage.md

@@ -10,7 +10,7 @@ Fork Stirling-PDF and create a new branch out of `main`.
 
 Then add a reference to the language in the navbar by adding a new language entry to the dropdown:
 
-- Edit the file: [languages.html](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/stirling-pdf/src/main/resources/templates/fragments/languages.html)
+- Edit the file: [languages.html](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/app/core/src/main/resources/templates/fragments/languages.html)
 
 
 For example, to add Polish, you would add:
@@ -25,7 +25,7 @@ The `data-bs-language-code` is the code used to reference the file in the next s
 
 Start by copying the existing English property file:
 
-- [messages_en_GB.properties](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/stirling-pdf/src/main/resources/messages_en_GB.properties)
+- [messages_en_GB.properties](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/app/core/src/main/resources/messages_en_GB.properties)
 
 Copy and rename it to `messages_{your data-bs-language-code here}.properties`. In the Polish example, you would set the name to `messages_pl_PL.properties`.
 
@@ -62,15 +62,15 @@ Make sure to place the entry under the correct language section. This helps main
 #### Windows command
 
 ```powershell
-python .github/scripts/check_language_properties.py --reference-file stirling-pdf\src\main\resources\messages_en_GB.properties --branch "" --files stirling-pdf\src\main\resources\messages_pl_PL.properties
+python .github/scripts/check_language_properties.py --reference-file app\core\src\main\resources\messages_en_GB.properties --branch "" --files app\core\src\main\resources\messages_pl_PL.properties
 
-python .github/scripts/check_language_properties.py --reference-file stirling-pdf\src\main\resources\messages_en_GB.properties --branch "" --check-file stirling-pdf\src\main\resources\messages_pl_PL.properties
+python .github/scripts/check_language_properties.py --reference-file app\core\src\main\resources\messages_en_GB.properties --branch "" --check-file app\core\src\main\resources\messages_pl_PL.properties
 ```
 
 #### Linux command
 
 ```bash
-python3 .github/scripts/check_language_properties.py --reference-file stirling-pdf/src/main/resources/messages_en_GB.properties --branch "" --files stirling-pdf/src/main/resources/messages_pl_PL.properties
+python3 .github/scripts/check_language_properties.py --reference-file app/core/src/main/resources/messages_en_GB.properties --branch "" --files app/core/src/main/resources/messages_pl_PL.properties
 
-python3 .github/scripts/check_language_properties.py --reference-file stirling-pdf/src/main/resources/messages_en_GB.properties --branch "" --check-file stirling-pdf/src/main/resources/messages_pl_PL.properties
+python3 .github/scripts/check_language_properties.py --reference-file app/core/src/main/resources/messages_en_GB.properties --branch "" --check-file app/core/src/main/resources/messages_pl_PL.properties
 ```