Footnote Combobox UI

Goal

Replace the current overbuilt footnote definition editing block with a simpler footnote UX closer to Obsidian, and explore footnote insertion through an inline combobox that suggests the next identifier first while reusing existing combobox patterns.

Phases

• Gather learnings and existing footnote / combobox seams
• Define the target UX and minimal spec change
• Implement the UI/behavior change with tests first where sane
• Verify the live result in the browser

Notes

• Keep this scoped to current Plate footnote surfaces, not fantasy new note-link systems.
• Prefer reusing the emoji/combobox infrastructure over inventing another popup.
• The current pain points are:
  • the "Editor Definition" block feels too heavy
  • inserting refs should feel lighter and more inline
• Outcome:
  • footnote definitions now render as compact numbered rows instead of boxed cards with a visible "Back to reference" control
  • typing [^ on an empty line in the live docs editor opens a combobox with the next free id first
  • selecting the new-footnote row inserted [4] inline and created a compact 4 definition row below without jumping into a heavy definition card