feat: add heading breadcrumb bar and TOC panel to editor

[hoiyada7-maker]

Summary

• Adds a heading breadcrumb bar above the editor showing the current heading hierarchy (e.g. H1 > H2 > H3) in bold orange
• Clicking the bar opens a tree-structured TOC panel with fold/unfold controls per node
• Clicking a heading in the TOC navigates the editor to that line

Details

• New EditorPane component wraps Editor + HeadingBreadcrumb with cursor tracking
• onCursorLine callback added to Editor — fires on every selection/cursor change
• TOC renders as a tree: headings with children show ▾/▸ toggle; leaf nodes show a dot
• Each node independently collapsible in the TOC panel
• Current heading highlighted in orange; ancestor headings in muted orange
• Active item auto-scrolls into view when TOC opens
• Long heading text truncated to 15 chars per segment in the breadcrumb bar

Test plan

• Open a markdown file with multiple heading levels
• Verify breadcrumb bar shows full path (e.g. Introduction > Setup > Quick Start)
• Move cursor to different sections — breadcrumb updates in real time
• Click breadcrumb to open TOC panel
• Verify current heading is highlighted and scrolled into view
• Click ▾/▸ on a parent heading to collapse/expand its children
• Click a heading label to jump the editor to that line

🤖 Generated with Claude Code