Add gat-description modal when a Pi-Rat earns a Gat

Mirrors the name modal: earning the Gat objective sets
Player.needs_gat_description, and GatModal.svelte (rendered by Dashboard
across all phases) prompts for a one-of-a-kind Gat description, stored in
Player.gat_description and shown on the character sheet. Like a stolen
Name, the Gat's description travels with it through Gat Tax transfers.

- Player.gat_description / needs_gat_description (+ migration)
- toggle_objective personal_1 sets/clears the prompt
- crud_challenge gat-tax paths move the description with the Gat
- set-gat-description route; CharacterSheet displays it

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

TODO.md

@@ -20,7 +20,7 @@
 
 - [x] Add a toast pop-up for new events entering the event log, that then fades into the event log button
 
-- [ ] When you get a gat, there should be a pop up to enter a description of it, as with your name
+- [x] When you get a gat, there should be a pop up to enter a description of it, as with your name
 
 - [ ] In between scenes, there really only needs to be one panel with the roster and voting status. There doesn't need to be a whole extra panel to inform players that the deep will later get to refresh their hands.