draft outline: live heading list with click-to-jump

[jorgefilipecosta]

Summary

• Replaces the Outline tab placeholder with a live, clickable list of headings extracted from the editor's lezer markdown tree.
• Indents entries by heading level, marks the heading containing the cursor as active, and jumps the editor to the heading line on click.
• Updates on every doc change with no extra parsing — reuses the syntax tree the existing markdown decorations already build.

Why

The earlier stub limited long-form drafts: writers had no way to see structure or navigate by heading. This is the missing navigation surface.

Notes

• ATX headings only for now (# Heading). Setext (=== / ---) is one extra case away if needed.
• Code blocks are correctly skipped — the lezer parser doesn't emit heading nodes inside fenced code.
• This branch is stacked on the unmerged t3 polish work; reviewing the top two commits gives the outline diff (d976742 implementation, 975ac48 tests).

Test plan

• 15 unit tests over the lezer extractor, equality guard, and active-row math.
• e2e: populated panel with three levels, click-to-jump with active highlight, live updates on typing, empty placeholder.
• Manual: opened a long draft, switched to Outline, clicked through entries, verified scroll + cursor placement.