Write Mode: Add 'Incoming Jumps' section to LabelPropertiesPanel

[mikkisguy]

Summary

Show which labels jump TO the current label in Write Mode's LabelPropertiesPanel, providing writers with context about narrative flow.

Current State

• LabelPropertiesPanel shows "Outgoing Jump" (where this label goes)
• No visibility into "Incoming Jumps" (what comes to this label)

Proposed Change

Add a new collapsible section "Incoming Jumps" that displays:

• List of label names that jump to the current label
• For each incoming jump, show:
  • Label name/title
  • Context (menu choice, automatic jump, etc.)
  • Any conditions on that jump

Design

• Collapsible section (same pattern as other sections)
• Default closed to avoid visual clutter
• Small badges showing count (e.g., "3 incoming jumps")
• Click to expand for details

Technical Requirements

1. Backend: Add endpoint to query incoming jumps for a label
   • Store incoming jump references on label creation/updates
2. Frontend:
   • Fetch incoming jumps data
   • Display in LabelPropertiesPanel
   • Style consistent with existing sections

Implementation Notes

• Consider performance for projects with many labels
• Incoming jumps may include menu choices, automatic jumps, and conditional jumps

Acceptance Criteria

• Incoming Jumps section appears in LabelPropertiesPanel
• Shows list of labels that jump to the current label
• Collapsed by default with badge showing count
• Expands to show detailed information
• Performance acceptable for typical project sizes