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.

.devcontainer/devcontainer.json

@@ -119,7 +119,9 @@
                 "EditorConfig.EditorConfig", // EditorConfig support for maintaining consistent coding styles
                 "ms-azuretools.vscode-docker", // Docker extension for Visual Studio Code
                 "charliermarsh.ruff", // Ruff extension for Ruff language support
-                "github.vscode-github-actions" // GitHub Actions extension for Visual Studio Code
+                "github.vscode-github-actions", // GitHub Actions extension for Visual Studio Code
+                "stylelint.vscode-stylelint", // Stylelint extension for CSS and SCSS linting
+                "redhat.vscode-yaml" // YAML extension for Visual Studio Code
             ]
         }
     },

.github/labeler-config-srvaroa.yml

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

.vscode/extensions.json

@@ -17,5 +17,7 @@
     "GitHub.vscode-pull-request-github", // GitHub Pull Requests extension for Visual Studio Code
     "charliermarsh.ruff", // Ruff code formatter for Python to follow the Ruff Style Guide
     "yzhang.markdown-all-in-one", // Markdown All-in-One extension for enhanced Markdown editing
+    "stylelint.vscode-stylelint", // Stylelint extension for CSS and SCSS linting
+    "redhat.vscode-yaml", // YAML extension for Visual Studio Code
   ]
 }

.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",

devGuide/STYLELINT.md

@@ -0,0 +1,47 @@
+# STYLELINT.md
+
+## Usage
+
+Apply Stylelint to your project's CSS with the following steps:
+
+1. **NPM Script**
+
+   - Go to directory: `devTools/`
+
+   - Add Stylelint & stylistic/stylelint-plugin
+     ```bash
+     npm install --save-dev stylelint stylelint-config-standard
+     npm install --save-dev @stylistic/stylelint-plugin
+     ```
+   - Add a script entry to your `package.json`:
+     ```jsonc
+     {
+       "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"
+       }
+     }
+     ```
+   - Run the linter:
+     ```bash
+     npm run lint:css:check
+     npm run lint:css:fix
+     ```
+
+2. **CLI Usage**
+
+   - Lint all CSS files:
+     ```bash
+     npx stylelint ../stirling-pdf/src/main/**/*.css ../proprietary/src/main/resources/static/css/*.css
+     ```
+   - Lint a single file:
+     ```bash
+     npx stylelint ../proprietary/src/main/resources/static/css/audit-dashboard.css
+     ```
+   - Apply automatic fixes:
+     ```bash
+     npx stylelint "../stirling-pdf/src/main/**/*.css" "../proprietary/src/main/resources/static/css/*.css" --fix
+     ```
+
+For full configuration options and rule customization, refer to the official documentation: [https://stylelint.io](https://stylelint.io)
+

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"
+  }
+}