Unify linting: replace Stylelint and markdownlint with ESLint (#4148)

We were running three separate lint tools (ESLint, Stylelint,
markdownlint-cli2) and a separate Prettier step. This PR consolidates
everything: `@eslint/css` and `@eslint/markdown` bring CSS and Markdown
linting into ESLint, so one tool now covers JS, CSS, and Markdown. The
Prettier step got merged into the lint scripts too, so there's now just
`test:lint` to check things and `lint:fix` to fix them.

Outcome: fewer tools to maintain, simpler/fewer scripts, fewer config
files.

As a side effect, the dependabot alerts
https://github.com/MagicMirrorOrg/MagicMirror/security/dependabot/140
and
https://github.com/MagicMirrorOrg/MagicMirror/security/dependabot/141
are resolved, as we no longer use the vulnerable `fast-uri` package
(which was a dependency of `stylelint`).

.github/CONTRIBUTING.md

@@ -8,25 +8,10 @@ We hold our code to standard, and these standards are documented below.
 
 We use [prettier](https://prettier.io/) for automatic formatting a lot all our files. The configuration is in our `prettier.config.mjs` file.
 
-To run prettier, use `node --run lint:prettier`.
+And we use [ESLint](https://eslint.org) to lint our JavaScript, Markdown, and CSS files. The configuration is in our `eslint.config.mjs` file.
 
-### JavaScript: Run ESLint
-
-We use [ESLint](https://eslint.org) to lint our JavaScript files. The configuration is in our `eslint.config.mjs` file.
-
-To run ESLint, use `node --run lint:js`.
-
-### CSS: Run StyleLint
-
-We use [StyleLint](https://stylelint.io) to lint our CSS. The configuration is in our `stylelint.config.mjs` file.
-
-To run StyleLint, use `node --run lint:css`.
-
-### Markdown: Run markdownlint
-
-We use [markdownlint-cli2](https://github.com/DavidAnson/markdownlint-cli2) to lint our markdown files. The configuration is in our `.markdownlint.json` file.
-
-To run markdownlint, use `node --run lint:markdown`.
+To check for formatting and linting errors, use `node --run test:lint`
+To fix formatting and linting errors, use `node --run lint:fix`.
 
 ## Testing
 
@@ -43,6 +28,7 @@ The `package.json` scripts expose finer-grained test commands:
 - `test:watch` – keep Vitest in watch mode for fast local feedback
 - `test:ui` – open the Vitest UI dashboard (needs OS file-watch support enabled)
 - `test:calendar` – run the legacy calendar debug helper
-- `test:css`, `test:markdown`, `test:prettier`, `test:spelling`, `test:js` – lint-only scripts that enforce formatting, spelling, markdown style, and ESLint.
+- `test:lint` – run linter and formatter checks
+- `test:spelling` – run the spell checker
 
 You can invoke any script with `node --run <script>` (or `npm run <script>`). Individual files can still be targeted directly, e.g. `npx vitest run tests/e2e/env_spec.js`.

.github/PULL_REQUEST_TEMPLATE.md

@@ -10,7 +10,7 @@ Hello and thank you for wanting to contribute to the MagicMirror² project!
 > - What does the pull request accomplish? Use a list if needed.
 > - If it includes major visual changes please add screenshots.
 >
-> 3. Please run `node --run lint:prettier` before submitting so that
+> 3. Please run `node --run lint:fix` before submitting so that
 >    style issues are fixed.
 
 **Note**: Sometimes the development moves very fast. It is highly

.github/workflows/automated-tests.yaml

@@ -33,10 +33,7 @@ jobs:
           node --run install-mm:dev
       - name: "Run linter tests"
         run: |
-          node --run test:prettier
-          node --run test:js
-          node --run test:css
-          node --run test:markdown
+          node --run test:lint
   test:
     runs-on: ubuntu-24.04
     timeout-minutes: 30