chore: add integrate Stylelint for CSS linting (#3909)

# Description of Changes

**What was changed**  
- Added a new `.stylelintrc.json` to configure Stylelint with
`stylelint-config-standard` and custom ignore rules.
- Created a `lint:css` script in `package.json` and added
`stylelint`/`stylelint-config-standard` to `devDependencies`.
  - Added `package-lock.json` to lock dependencies.  
- Updated numerous CSS files under
`stirling-pdf/src/main/resources/static/css/` to fix lint errors
(shorthand properties, removed redundant units, consistent box-shadow
syntax, margin shorthand, etc.).

**Why the change was made**  
- To enforce consistent, modern CSS code style across the project, catch
errors early, and enable automated fixing of common lint issues.


---

## 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.

.github/labeler-config-srvaroa.yml

@@ -126,6 +126,7 @@ labels:
       - '.pre-commit-config'
       - '.github/workflows/pre_commit.yml'
       - 'devGuide/.*'
+      - 'devTools/.*'
 
   - label: 'Test'
     files: