feat(ui5-checkbox): make component customizable

[NakataCode]

The checkbox now scales proportionally when font-size is set on the host — the inner box, icon, indeterminate dot, and display-only icon are all sized in em so they follow the scale. A few internal changes were needed to make this work correctly: overflow: hidden moved to .ui5-checkbox-root, padding-block is now always applied, and compact density overrides were added for the hardcoded em values. Default rendering is pixel-identical to before.

Customization examples:

/* Scale the entire component by setting font-size on the host */
  .custom {
      font-size: 20px;
  }

  /* Reduce the padding around the box via the root part */
  .custom::part(root) {
      padding: 4px;
  }

  /* Fix the focus ring to hug the new bounds after padding override */
  .custom::part(root):focus::before,
  .custom::part(root):focus-visible::before {
      inset: 0;
  }

JIRA: BGSOFUIPIRIN-7002

[ui5-webcomponents-bot]

🧹 Preview deployment cleaned up: https://pr-13617--ui5-webcomponents.netlify.app