Implement the 008-persist-encounter feature that saves encounter state to localStorage so it survives page reloads

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/persistence/encounter-storage.ts

@@ -0,0 +1,63 @@
+import {
+	combatantId,
+	createEncounter,
+	type Encounter,
+	isDomainError,
+} from "@initiative/domain";
+
+const STORAGE_KEY = "initiative:encounter";
+
+export function saveEncounter(encounter: Encounter): void {
+	try {
+		localStorage.setItem(STORAGE_KEY, JSON.stringify(encounter));
+	} catch {
+		// Silently swallow errors (quota exceeded, storage unavailable)
+	}
+}
+
+export function loadEncounter(): Encounter | null {
+	try {
+		const raw = localStorage.getItem(STORAGE_KEY);
+		if (raw === null) return null;
+
+		const parsed: unknown = JSON.parse(raw);
+
+		if (typeof parsed !== "object" || parsed === null || Array.isArray(parsed))
+			return null;
+
+		const obj = parsed as Record<string, unknown>;
+
+		if (!Array.isArray(obj.combatants)) return null;
+		if (typeof obj.activeIndex !== "number") return null;
+		if (typeof obj.roundNumber !== "number") return null;
+
+		const combatants = obj.combatants as unknown[];
+		for (const c of combatants) {
+			if (typeof c !== "object" || c === null || Array.isArray(c)) return null;
+			const entry = c as Record<string, unknown>;
+			if (typeof entry.id !== "string") return null;
+			if (typeof entry.name !== "string") return null;
+		}
+
+		const rehydrated = combatants.map((c) => {
+			const entry = c as Record<string, unknown>;
+			return {
+				id: combatantId(entry.id as string),
+				name: entry.name as string,
+				initiative:
+					typeof entry.initiative === "number" ? entry.initiative : undefined,
+			};
+		});
+
+		const result = createEncounter(
+			rehydrated,
+			obj.activeIndex,
+			obj.roundNumber,
+		);
+		if (isDomainError(result)) return null;
+
+		return result;
+	} catch {
+		return null;
+	}
+}