Allow custom chat reaction icons

[mpiorowski]

Summary

• Chat reactions now store the actual icon text instead of a fixed numeric reaction kind, enabling custom icon-picker reactions while preserving the existing quick reaction flow.

Changes

• Migrates chat_message_reactions.kind to chat_message_reactions.icon, mapping existing reactions to their current emoji labels.
• Keeps f then 1-9 as quick reactions and changes f then 0 to open the icon picker for any catalog icon.
• Updates reaction rendering, owner overlays, help text, service events, and reaction tests to use icon strings.
• Adds DRAGON.md with exploratory notes for a possible original daily door RPG.

Behavior notes

• Existing reactions are migrated to equivalent emoji text.
• Each user still has at most one reaction per message.
• f then 0 no longer quick-reacts with the wave emoji; it opens custom icon selection.

Feature flags

None

Screenshots / Video

Not included in this draft; attach before review.

Testing

• Automated: Updated existing chat reaction model/service/input/UI tests for icon-backed reactions and custom reaction picker behavior. Not run locally per repo policy.
• Manual: Not run.