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>

frontend/src/components/scene/CharacterSheet.svelte

@@ -80,6 +80,9 @@
             <p><strong>Look:</strong> {state.player.avatar_look}</p>
             <p><strong>Smell:</strong> {state.player.avatar_smell}</p>
             <p><strong>First Words:</strong> "{state.player.first_words}"</p>
+            {#if state.player.gat_description}
+                <p><strong>🔫 Gat:</strong> {state.player.gat_description}</p>
+            {/if}
         </div>
 
         {#if state.player.other_like || state.player.other_hate}