feat(outline): highlight active heading on scroll

[adibhanna]

Summary

• Tracks the topmost-visible heading from the editor's scroll surface (or the preview's, in preview mode) and highlights the matching row in the outline panel so users can orient at a glance while scrolling long notes.
• Probe sits ~25% down the viewport (capped at 140px) so a heading lights up as soon as it enters the upper portion of the visible area, not only after it scrolls past the very top edge.
• Active row uses an accent tint (bg-accent/12, text-accent, semi-bold) and auto-scrolls into view with block: 'nearest' so it doesn't fight the user when already visible.

Test plan

• Open a long note with multiple headings, toggle the outline panel, and scroll the editor — the active heading should update as each new section enters the upper portion of the viewport.
• Scroll back to the top above all headings — no row should be highlighted.
• Switch the pane to Preview mode and scroll there — the active heading still updates correctly.
• In Split mode, scroll the editor side — outline tracks the editor.
• Type new headings inline; line numbers shift but the active row stays correct.
• Filter the outline so the active heading is filtered out — no errors, other rows render normally.
• Note with zero headings — no highlight, no errors.