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/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