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/turn-navigation.tsx

@@ -15,7 +15,7 @@ export function TurnNavigation({
 	onAdvanceTurn,
 	onRetreatTurn,
 	onClearEncounter,
-}: TurnNavigationProps) {
+}: Readonly<TurnNavigationProps>) {
 	const hasCombatants = encounter.combatants.length > 0;
 	const isAtStart = encounter.roundNumber === 1 && encounter.activeIndex === 0;
 	const activeCombatant = encounter.combatants[encounter.activeIndex];