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/bulk-import-prompt.tsx

@@ -18,7 +18,7 @@ export function BulkImportPrompt({
 	importState,
 	onStartImport,
 	onDone,
-}: BulkImportPromptProps) {
+}: Readonly<BulkImportPromptProps>) {
 	const [baseUrl, setBaseUrl] = useState(DEFAULT_BASE_URL);
 	const baseUrlId = useId();
 	const totalSources = getAllSourceCodes().length;