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/source-fetch-prompt.tsx

@@ -18,7 +18,7 @@ export function SourceFetchPrompt({
 	fetchAndCacheSource,
 	onSourceLoaded,
 	onUploadSource,
-}: SourceFetchPromptProps) {
+}: Readonly<SourceFetchPromptProps>) {
 	const [url, setUrl] = useState(() => getDefaultFetchUrl(sourceCode));
 	const [status, setStatus] = useState<"idle" | "fetching" | "error">("idle");
 	const [error, setError] = useState<string>("");