refactor(theme): remove hard-coded tag border-radius

[homotechsual]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

.tagRegular has a fixed 0.5REM border radius - it's the only place I've found that doesn't inherit from --ifm-global-radius without a good reason.

Test Plan

Default behaviour here will be no change unless --ifm-global-radius is customised in which case tag elements will now inherit.

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 64	🟢 98	🟢 100	🟢 100	🟠 80	Report
/docs/installation	🟠 69	🟢 100	🟢 100	🟢 100	🟢 90	Report

[slorber]

LGTM 👍 this reduces a bit the radius (0.5rem -> 0.4rem) but actually looks better