fix(accordion): use z-index overlap for themed open item borders (alternative to #42092)

[q121212]

Closes #42092

Problem

When a themed accordion item is opened, the top border appears gray instead of the theme color. This happens because the previous (closed) item retains a gray border-bottom, and the open item has no border-top (from the &:not(:first-of-type) { border-block-start: 0; } rule).

Approach (per @mdo's review suggestion)

Instead of manipulating borders with :has() and explicit border-block-start, use relative positioning on the open item:

1. position: relative; z-index: 2 — lifts the open item above siblings
2. margin-block-start: calc(-1 * var(--accordion-border-width)) — pulls it up by one border-width, visually overlapping the previous item's bottom border

This is simpler than PR #42092's approach:

• No :has() dependency (better browser support)
• Only 3 properties added
• Naturally works with all variants (flush, always-open, theme-*)

Testing

• npm run css-lint passes
• npm run css builds cleanly
• Visual: open accordion with .theme-primary shows correct colored top border
• Visual: flush variant unchanged
• Visual: always-open variant unchanged