feat(mjml-social): add border attribute to mj-social-element

[danfitz36]

Fixes #2466.

Email accessibility tooling (Email on Acid, Litmus) flags <img> tags in social blocks for missing border attributes. mj-image already supports a border attribute that renders as CSS on the resulting <img>; mj-social-element did not, leaving no way to set or default it without copying the rendered HTML out by hand.

Changes

• packages/mjml-social/src/SocialElement.js
  • Add border: 'string' to allowedAttributes
  • Add border: '0' to defaultAttributes (matches mj-image's default)
  • Add border: this.getAttribute('border') to the img styles in getStyles()
• packages/mjml-social/src/Social.js
  • Add border: 'string' to allowedAttributes
  • Add border to the cascade list in getSocialElementAttributes so a single declaration on mj-social applies to every child. Element-level borders still take precedence.
• packages/mjml-social/README.md
  • New border rows in both attribute tables
• packages/mjml/test/social-border.test.js (new)
  • Four cases: default 0 on every element, explicit element override, parent cascade, and element-overrides-cascade

Sample rendered output

With:

<mj-social border="1px solid #ccc">
  <mj-social-element name="facebook" href="https://mjml.io/" />
  <mj-social-element name="x" href="https://mjml.io/" border="2px solid red" />
  <mj-social-element name="linkedin" href="https://mjml.io/" />
</mj-social>

The three <img> tags emit:

• style="border:1px solid #ccc;border-radius:3px;display:block;" (cascaded)
• style="border:2px solid red;border-radius:3px;display:block;" (element override)
• style="border:1px solid #ccc;border-radius:3px;display:block;" (cascaded)

Verification

• yarn build ✓ all packages
• yarn test ✓ 74 → 78 (the 4 new cases)
• Lint clean on changed files