Footnotes you can follow: hover preview + jump between reference and definition

[tosinamuda]

User story
As an academic writer, I want to hover a footnote reference to read it and click to jump between reference and definition, so that footnote-heavy chapters are navigable (today references render styled but inert).

Acceptance criteria

• Hovering [^ref] shows a tooltip with the definition's rendered text
• Cmd+click a reference jumps to its definition; Cmd+click a definition jumps back to (the first) reference; jumps land exactly (honest heights, fix(editor): scroll jump on select after fast scroll — honest widget height estimates #120)
• Unresolved references (no matching definition) get a distinct style — the writer sees the dangling footnote
• Works in rich and raw modes, and in external files (Compose as default markdown opener — with a deliberate landing policy (Drafts vs in-place) #113)
• WebKit browser-tier test for hover + jump

Sub-tasks

• Definition index (extend footnotePlugin's existing scan)
• CM6 hoverTooltip + rendered preview
• Jump commands (ref→def, def→ref) + click wiring
• Unresolved-ref styling + tests