Fix embedded form dropdown scroll propagation

[jackmusick]

Fixes #564.

What changed

• constrain cmdk selected-item scrolling to the nearest command list instead of allowing native scrollIntoView to traverse the iframe boundary
• focus the post-submit confirmation with preventScroll
• add a self-contained cross-origin iframe regression for the consecutive Company Size / How did you hear about us sequence

Evidence

The live headed Chrome reproduction moved the host page 646-811px. Runtime isolation showed suppressing focus did not stop it, while suppressing cmdk selected-item scrollIntoView (or scoping it to the nearest container) did. This matches the upstream cmdk report and proposed fix:

• Off-screen <Command /> mount can scroll the window unexpectedly dip/cmdk#405
• fix: guard scrollSelectedIntoView against viewport scroll dip/cmdk#408

The local implementation preserves keyboard navigation by adjusting only the command list scrollTop, including long-list upward and downward movement.

Focused verification

• TypeScript build: passed
• ESLint on touched files: passed
• Command, FormConfirmation, and form MultiCombobox tests: 11 passed
• Cross-origin physical-click Playwright regression: passed 3/3 consecutive repetitions

Per request, the broad repository test suites were not run locally and are left to CI.