Match the Share rail to the design and add the 40px block spacer

Problem/Motivation

• The Share rail does not match the design.
• The design system's 40px block rhythm has no Bootstrap spacer step (the scale jumps 24px → 48px).
• The section header/footer slots use 24px where the design specifies 40px.

Steps to reproduce

1. Load a Canvas page on Horizon Aid that renders a section with header/footer slots and the Share rail.
2. Compare the header/footer slot spacing and the Share rail styling against the design system.
3. Note the section uses mb-4/mt-4 (24px) where the design specifies 40px.

Proposed resolution

• Retune the Share component's tokens (icon size, button size, gap, background, radius, colour) to match the design's pill style.
• Add a 40px block step to $spacers, generating *-block utilities (mt-block, mb-block, …).
• Point the section header/footer slots at mb-block/mt-block.

Note: the theme's Sass variables file loads before Bootstrap defines $spacers and $spacer, so the new step has to be declared with literal rem values, not by referencing $spacer.

Known gap, out of scope here: the design shows four Share icons but only three render; the set/order comes from the Webshare module's configuration, not this theme — needs its own separate issue.

This issue unblocks the Horizon Aid recipe MR !17, which relies on the new mb-block utility class.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ✅ Update Release Notes
• ✅ Release

User interface changes

• The Share rail renders as the design's circular pills.
• The 40px block rhythm applies site-wide, across every Canvas section, not just this one component.

API changes

• $spacers gains a block step, so *-block utility classes (e.g. mb-block, py-block) become available.

Data model changes

• None.

Release notes snippet

• The Share rail now matches the design, and the design system's 40px block spacing is available site-wide as *-block utilities.