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>

packages/domain/src/undo-redo.ts

@@ -0,0 +1,70 @@
+import type { DomainError, Encounter } from "./types.js";
+
+export interface UndoRedoState {
+	readonly undoStack: readonly Encounter[];
+	readonly redoStack: readonly Encounter[];
+}
+
+const MAX_UNDO_STACK = 50;
+
+export const EMPTY_UNDO_REDO_STATE: UndoRedoState = {
+	undoStack: [],
+	redoStack: [],
+};
+
+export function pushUndo(
+	state: UndoRedoState,
+	snapshot: Encounter,
+): UndoRedoState {
+	const newStack = [...state.undoStack, snapshot];
+	if (newStack.length > MAX_UNDO_STACK) {
+		newStack.shift();
+	}
+	return { undoStack: newStack, redoStack: [] };
+}
+
+export function undo(
+	state: UndoRedoState,
+	currentEncounter: Encounter,
+): { state: UndoRedoState; encounter: Encounter } | DomainError {
+	if (state.undoStack.length === 0) {
+		return {
+			kind: "domain-error",
+			code: "nothing-to-undo",
+			message: "Nothing to undo",
+		};
+	}
+	const restored = state.undoStack.at(-1) as Encounter;
+	return {
+		state: {
+			undoStack: state.undoStack.slice(0, -1),
+			redoStack: [...state.redoStack, currentEncounter],
+		},
+		encounter: restored,
+	};
+}
+
+export function redo(
+	state: UndoRedoState,
+	currentEncounter: Encounter,
+): { state: UndoRedoState; encounter: Encounter } | DomainError {
+	if (state.redoStack.length === 0) {
+		return {
+			kind: "domain-error",
+			code: "nothing-to-redo",
+			message: "Nothing to redo",
+		};
+	}
+	const restored = state.redoStack.at(-1) as Encounter;
+	return {
+		state: {
+			undoStack: [...state.undoStack, currentEncounter],
+			redoStack: state.redoStack.slice(0, -1),
+		},
+		encounter: restored,
+	};
+}
+
+export function clearHistory(): UndoRedoState {
+	return EMPTY_UNDO_REDO_STATE;
+}