fix: sync aria-activedescendant with selection

[wanxiankai]

Summary

• sync selectedItemId from cmdk's item registry when the selected value changes
• keep the active descendant id available when matching items register during initial render
• add Playwright coverage for initial and filtered selection aria-activedescendant

Fixes #373

Tests

• pnpm build
• pnpm test test/basic.test.ts
• pnpm test:format