feat: granular export options for sharing adventures

[Flo0806]

Summary

When exporting a campaign for sharing (e.g., via the Store), users should have granular control over what to include beyond just "Full" vs "Partial" export.

Current Behavior

• Full Export: Exports everything (entities, calendar, maps, sessions, currencies, notes, pinboard)
• Partial Export: Only selected entities with their relations/images/documents

Proposed Enhancement

Replace the binary Full/Partial choice with a checkbox-based selection:

☑ Entities (NPCs, Locations, Items, Factions, Lore, Players)
  ☐ Only selected entities
  
☑ Calendar & Currencies
☑ Maps with Markers & Areas
☐ Sessions (for backup/personal use)
☐ Notes & Pinboard (personal)

Use Cases

Scenario	What to include
Share adventure via Store	Entities, Calendar, Maps
Share One-Shot	Entities, Maps (no calendar)
Personal backup	Everything including Sessions
Share a few NPCs	Only selected entities

Future Consideration: Adventure Chapters

Sessions are designed for tracking "what happened" in play sessions. For shared adventures, a dedicated "Adventure Chapters" or "Scenes" feature might be more appropriate:

• Title, Description (what SHOULD happen)
• Linked entities (relevant NPCs, Locations, Items)
• Sort order
• Optional: estimated duration, difficulty

This would provide a cleaner separation between:

• Adventure structure (for sharing)
• Play sessions (personal tracking)

Implementation Notes

• Export dialog needs redesign with checkboxes
• Backend already supports all data types
• Default presets could simplify common scenarios ("For Store", "Full Backup", "Quick Share")