Make player character color and icon optional

Clicking an already-selected color or icon in the create/edit form now
deselects it. PCs without a color use the default combatant styling;
PCs without an icon show no icon. Domain, application, persistence,
and display layers all updated to handle the optional fields.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/App.tsx

@@ -431,8 +431,8 @@ export function App() {
 							name,
 							ac,
 							maxHp,
-							color,
-							icon,
+							color: color ?? null,
+							icon: icon ?? null,
 						});
 					} else {
 						createPlayerCharacter(name, ac, maxHp, color, icon);