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/components/icon-grid.tsx

@@ -19,7 +19,7 @@ export function IconGrid({ value, onChange }: IconGridProps) {
 					<button
 						key={iconId}
 						type="button"
-						onClick={() => onChange(iconId)}
+						onClick={() => onChange(value === iconId ? "" : iconId)}
 						className={cn(
 							"flex h-9 w-9 items-center justify-center rounded-md transition-all",
 							value === iconId