Use Readonly props and optional chaining/nullish coalescing

Mark component props as Readonly<> across 15 component files and
simplify edit-player-character field access with optional chaining
and nullish coalescing.

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

packages/domain/src/edit-player-character.ts

@@ -23,7 +23,7 @@ interface EditFields {
 }
 
 function validateFields(fields: EditFields): DomainError | null {
-	if (fields.name !== undefined && fields.name.trim() === "") {
+	if (fields.name?.trim() === "") {
 		return {
 			kind: "domain-error",
 			code: "invalid-name",
@@ -81,9 +81,9 @@ function applyFields(
 ): PlayerCharacter {
 	return {
 		id: existing.id,
-		name: fields.name === undefined ? existing.name : fields.name.trim(),
-		ac: fields.ac === undefined ? existing.ac : fields.ac,
-		maxHp: fields.maxHp === undefined ? existing.maxHp : fields.maxHp,
+		name: fields.name?.trim() ?? existing.name,
+		ac: fields.ac ?? existing.ac,
+		maxHp: fields.maxHp ?? existing.maxHp,
 		color:
 			fields.color === undefined
 				? existing.color