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-toasts.tsx

@@ -11,7 +11,7 @@ export function BulkImportToasts({
 	state,
 	visible,
 	onReset,
-}: BulkImportToastsProps) {
+}: Readonly<BulkImportToastsProps>) {
 	if (!visible) return null;
 
 	if (state.status === "loading") {