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.

devTools/.stylelintrc.json

@@ -0,0 +1,69 @@
+{
+  "extends": [
+    "stylelint-config-standard"
+  ],
+  "plugins": [
+    "@stylistic/stylelint-plugin"
+  ],
+  "ignoreFiles": [
+    "stirling-pdf/src/main/resources/static/css/bootstrap*.css",
+    "stirling-pdf/src/main/resources/static/css/cookieconsent.css",
+    "stirling-pdf/src/main/resources/static/css/cookieconsentCustomisation.css",
+    "stirling-pdf/src/main/resources/static/css/prism.css",
+    "stirling-pdf/src/main/resources/static/pdfjs-legacy/**/*.css"
+  ],
+  "rules": {
+    "property-no-vendor-prefix": null,
+    "value-no-vendor-prefix": null,
+    "selector-no-vendor-prefix": null,
+    "media-feature-name-no-vendor-prefix": null,
+    "value-keyword-case": null,
+    "color-function-notation": null,
+    "alpha-value-notation": null,
+    "color-function-alias-notation": null,
+    "selector-class-pattern": null,
+    "selector-id-pattern": null,
+    "declaration-block-no-redundant-longhand-properties": null,
+    "media-feature-range-notation": "prefix",
+    "selector-attribute-quotes": null,
+    "at-rule-no-vendor-prefix": null,
+    "selector-not-notation": null,
+    "no-duplicate-selectors": [
+      true,
+      {
+        "disableFix": true
+      }
+    ],
+    "comment-word-disallowed-list": null,
+    "custom-property-pattern": null,
+    "no-descending-specificity": null,
+    "keyframes-name-pattern": null,
+    "comment-empty-line-before": [
+      "always",
+      {
+        "ignore": [
+          "stylelint-commands"
+        ]
+      }
+    ],
+    "block-no-empty": true,
+    "@stylistic/declaration-bang-space-after": "never",
+    "@stylistic/declaration-bang-space-before": "always",
+    "@stylistic/declaration-block-trailing-semicolon": "always",
+    "@stylistic/function-comma-space-after": [
+      "always-single-line",
+      {
+        "disableFix": false
+      }
+    ],
+    "@stylistic/function-comma-space-before": "never",
+    "@stylistic/color-hex-case": "lower",
+    "@stylistic/declaration-block-semicolon-newline-after": "always",
+    "@stylistic/indentation": [
+      2,
+      {
+        "baseIndentLevel": 2
+      }
+    ]
+  }
+}

devTools/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "stirling-pdf",
+  "version": "1.0.0",
+  "scripts": {
+    "lint:css:check": "stylelint \"../stirling-pdf/src/main/**/*.css\" \"../proprietary/src/main/resources/static/css/*.css\" --config .stylelintrc.json",
+    "lint:css:fix": "stylelint \"../stirling-pdf/src/main/**/*.css\" \"../proprietary/src/main/resources/static/css/*.css\" --config .stylelintrc.json --fix"
+  },
+  "devDependencies": {
+    "@stylistic/stylelint-plugin": "^3.1.3",
+    "stylelint": "^16.21.1",
+    "stylelint-config-standard": "^38.0.0"
+  }
+}