[SandObserver]

Summary

• Replace the unit list type in formatList with plain: a narrow conjunction list. CLDR's Chinese unit pattern joins items with nothing, which ran screen-reader labels together.
• Use formatList(..., 'plain') for the GitHub pull request filter label, the Connections map spoken lists and legend overflow title, and the System Summary screen-reader summary, in place of hard-coded commas or unit lists.

Testing
All green, e2e included. Added 2 tests.

Changelog

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.