Add undo/redo for all encounter actions

Memento-based undo/redo with full encounter snapshots. Undo stack
capped at 50 entries, persisted to localStorage. Triggered via
buttons in the top bar (inboard of turn navigation) and keyboard
shortcuts (Ctrl+Z / Ctrl+Shift+Z, Cmd on Mac, case-insensitive key
matching). Clear encounter resets both stacks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -108,45 +108,44 @@ function isValidCombatantEntry(c: unknown): boolean {
 	return typeof entry.id === "string" && typeof entry.name === "string";
 }
 
+export function rehydrateEncounter(parsed: unknown): Encounter | null {
+	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[];
+
+	// Handle empty encounter (cleared state) directly — createEncounter rejects empty arrays
+	if (combatants.length === 0) {
+		return {
+			combatants: [],
+			activeIndex: 0,
+			roundNumber: 1,
+		};
+	}
+
+	if (!combatants.every(isValidCombatantEntry)) return null;
+
+	const rehydrated = combatants.map(rehydrateCombatant);
+
+	const result = createEncounter(rehydrated, obj.activeIndex, obj.roundNumber);
+	if (isDomainError(result)) return null;
+
+	return result;
+}
+
 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[];
-
-		// Handle empty encounter (cleared state) directly — createEncounter rejects empty arrays
-		if (combatants.length === 0) {
-			return {
-				combatants: [],
-				activeIndex: 0,
-				roundNumber: 1,
-			};
-		}
-
-		if (!combatants.every(isValidCombatantEntry)) return null;
-
-		const rehydrated = combatants.map(rehydrateCombatant);
-
-		const result = createEncounter(
-			rehydrated,
-			obj.activeIndex,
-			obj.roundNumber,
-		);
-		if (isDomainError(result)) return null;
-
-		return result;
+		return rehydrateEncounter(parsed);
 	} catch {
 		return null;
 	}

apps/web/src/persistence/undo-redo-storage.ts

@@ -0,0 +1,45 @@
+import type { Encounter, UndoRedoState } from "@initiative/domain";
+import { EMPTY_UNDO_REDO_STATE } from "@initiative/domain";
+import { rehydrateEncounter } from "./encounter-storage.js";
+
+const UNDO_KEY = "initiative:encounter:undo";
+const REDO_KEY = "initiative:encounter:redo";
+
+export function saveUndoRedoStacks(state: UndoRedoState): void {
+	try {
+		localStorage.setItem(UNDO_KEY, JSON.stringify(state.undoStack));
+		localStorage.setItem(REDO_KEY, JSON.stringify(state.redoStack));
+	} catch {
+		// Silently swallow errors (quota exceeded, storage unavailable)
+	}
+}
+
+function loadStack(key: string): readonly Encounter[] {
+	try {
+		const raw = localStorage.getItem(key);
+		if (raw === null) return [];
+
+		const parsed: unknown = JSON.parse(raw);
+		if (!Array.isArray(parsed)) return [];
+
+		const valid: Encounter[] = [];
+		for (const entry of parsed) {
+			const rehydrated = rehydrateEncounter(entry);
+			if (rehydrated !== null) {
+				valid.push(rehydrated);
+			}
+		}
+		return valid;
+	} catch {
+		return [];
+	}
+}
+
+export function loadUndoRedoStacks(): UndoRedoState {
+	const undoStack = loadStack(UNDO_KEY);
+	const redoStack = loadStack(REDO_KEY);
+	if (undoStack.length === 0 && redoStack.length === 0) {
+		return EMPTY_UNDO_REDO_STATE;
+	}
+	return { undoStack, redoStack };
+}