fix: restore icon centering after MUI v9 upgrade

[swkatmask]

Summary

• Icons with dynamic color support (s: true, svg rendered as a child of the icon span — e.g. Icons.MaskBlue) drifted a few px down inside text-sized containers after the MUI v9 upgrade.
• Root cause: __createIcon passed fontSize: 0 as a system prop to Box. MUI v5 converted system props to CSS (font-size: 0), zeroing the strut so the svg filled the span. Since 52d5bce (5.15.20 → 9.2.0), Box only styles through sx — the prop leaked to the DOM as a useless font-size="0" attribute, the strut came back, and the inline svg got pushed below the baseline. Smaller size made it more visible (e.g. MaskBlue size={16} in the ProfileCard avatar badge sitting in an IconButton with font-size: 24px).
• Fix in packages/icons/utils/internal.js:
  • move fontSize: 0 into the sx base style (restores v5 semantics under v9);
  • add lineHeight: 0 to guard against host pages inheriting an absolute line-height;
  • drop the dead prop (also removes the React dev warning about the unrecognized fontSize prop).

Impact

__createIcon is the single factory for all generated icons, so this fixes every s: true icon at once. Background-image variants (no child element) are unaffected by fontSize/lineHeight. The SvgIcon-based createIcon() in utils/index.js is unaffected.

Verification

• Rebuilt dev extension, reloaded x.com profile page and inspected the injected shadow-root UI:
  • icon spans compute font-size: 0px; line-height: 0px; no font-size DOM attribute remains;
  • MaskBlue size={16} in the avatar badge: svg center vs button center delta = 0px (was offset before);
  • MaskBlue size={24} dialog header icon likewise centered;
  • console no longer logs the React does not recognize the "fontSize" prop warning.
• eslint on the changed file passes; commitlint passes.