Fix heading-order accessibility: configurable heading level on the Share component

Problem/Motivation

The Share (webshare) molecule in the Educare theme had the same axe-core WCAG 2.4.6 (Headings and Labels) heading-order violation as the Horizon Aid theme: its title always renders as a hardcoded <h3>, regardless of the heading level the surrounding section actually needs, so it breaks the document outline whenever the block sits under anything other than an <h2> section. This was already fixed on Educare in #3614488 (merged as MR !22). Horizon Aid's Share molecule is the same webshare:share override pattern with the identical hardcoded <h3>, so it needs the same fix.

Steps to reproduce

1. Install the Horizon Aid site template, or open any Horizon Aid page that renders a Share block.
2. Run an axe-core heading-order check on that page (e.g. the Varbase functional testing suite's a11y step, or axe DevTools).
3. Observe the heading-order violation: the Share block's <h3> title does not follow the heading level of the section it is placed in.

Proposed resolution

Decouple the heading LEVEL from the visual SIZE, mirroring the Educare fix exactly:

• Add a heading_level prop to components/molecules/share/share.component.yml (integer, enum 2-6, default 2).
• Use it in share.twig: <h{{ heading_level }}> instead of the hardcoded <h3>.
• Visual size stays fixed via the existing webshare__heading class, so callers can set the correct semantic level without changing how the title looks.

Merged as MR !9.

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

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Fixed a heading-order accessibility issue on the Share component by making its heading level configurable.