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>

apps/web/src/components/create-player-modal.tsx

@@ -24,7 +24,7 @@ export function CreatePlayerModal({
 	onClose,
 	onSave,
 	playerCharacter,
-}: CreatePlayerModalProps) {
+}: Readonly<CreatePlayerModalProps>) {
 	const [name, setName] = useState("");
 	const [ac, setAc] = useState("10");
 	const [maxHp, setMaxHp] = useState("10");