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.

.vscode/settings.json

@@ -9,6 +9,9 @@
   "[jsonc]": {
     "editor.defaultFormatter": "vscode.json-language-features"
   },
+  "[css]": {
+    "editor.defaultFormatter": "stylelint.vscode-stylelint"
+  },
   "[json]": {
     "editor.defaultFormatter": "vscode.json-language-features"
   },
@@ -27,6 +30,9 @@
   "[gradle]": {
     "editor.defaultFormatter": "vscjava.vscode-gradle"
   },
+  "[yaml]": {
+    "editor.defaultFormatter": "redhat.vscode-yaml"
+  },
   "java.compile.nullAnalysis.mode": "automatic",
   "java.configuration.updateBuildConfiguration": "interactive",
   "java.format.enabled": true,
@@ -119,6 +125,7 @@
   "html.format.indentHandlebars": true,
   "html.format.preserveNewLines": true,
   "html.format.maxPreserveNewLines": 2,
+  "stylelint.configFile": "devTools/.stylelintrc.json",
   "java.project.sourcePaths": [
     "stirling-pdf/src/main/java",
     "common/src/main/java",