[SandObserver]

Summary

• Inset each .dyn-page-grid by the badge overhang (7px scaled by --sc, clamped to the box padding) and give the page its own overflow:hidden, so a badge is clipped by the page it belongs to instead of by the shared viewport.
• Size the clip, the strip width, and the translateX step in the padded page width so pages no longer sit flush against each other.
• Take the inset out of the folder box padding and the dots row padding, leaving icon spacing, box size, and dot position unchanged.
• Give both folder overlays tabindex="-1" and pass each as initialFocus, so opening a folder no longer moves focus onto the first app link.
• Reset the plain :focus outline on both overlays and their app links, leaving the existing :focus-visible accent ring as the only focus style.

Testing
All green, except the Docker build smoke test, which has no local Docker and is left to CI. Added 10 tests in ui/test/folder-overlay.test.mjs covering the page inset, the padded page step, the per-page clip, and the focus target of both overlays.

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.
• CHANGELOG.md updated under Unreleased if the change is user-visible.