Adopt @wagtail/stylelint-config-wagtail v2 for CSS linting

[Srishti-j18]

Task of #743

Description

This updates bakerydemo to use @wagtail/stylelint-config-wagtail as the shared CSS linting baseline.

Changes in this PR:

• replace stylelint-config-standard with @wagtail/stylelint-config-wagtail
• renamed the file .stylelintrc to stylelint.config.mjs for consistency
• keep a small set of bakerydemo-specific rule overrides where the shared config is currently too strict for the existing stylesheet

Tested locally

I updated the frontend tooling packages with: npm install stylelint @wagtail/stylelint-config-wagtail --save-dev

I tested the updated Stylelint configuration against the project CSS locally.
Checks run:

  - npm run lint:css
  - npx stylelint bakerydemo/static/css/main.css
  - npx stylelint --fix bakerydemo/static/css/main.css

AI usage

I used GPT for the configuration definition and testing approach, and I manually applied the changes and verified them locally.

[thibaudcolas]

Looking spot on!