fix: correct outline active heading detection and scroll-to-center on click

[octo-patch]

Fixes #1039

Problem

Two bugs in the outline panel:

1. Wrong active heading highlight on scroll: The findActiveHeadingByScroll function iterated headings from first to last, returning the first heading whose position was at or above the viewport. With multiple headings scrolled past, this always returned the topmost heading instead of the most recently passed one, causing the highlight to be out of sync with the actual scroll position.

2. Heading not centered when clicked: Clicking a heading in the outline used editor.commands.scrollIntoView(), which only guarantees the selection is visible (often placing it at the bottom edge of the viewport) rather than centering it.

Solution

1. Changed findActiveHeadingByScroll to iterate in reverse order (last to first) so it returns the last heading whose top is at or above the viewport — i.e., the heading the user is currently reading.

2. Changed scrollToHeading to use domNode.scrollIntoView({ behavior: 'smooth', block: 'center' }) so the target heading is scrolled to the center of the editor viewport.

Testing

• Open a document with multiple headings spanning more than one screen
• Scroll through the document and verify the outline panel highlights the correct heading
• Click headings in the outline and verify they scroll to the center of the editor