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/application/src/ports.ts

@@ -3,6 +3,7 @@ import type {
 	CreatureId,
 	Encounter,
 	PlayerCharacter,
+	UndoRedoState,
 } from "@initiative/domain";
 
 export interface EncounterStore {
@@ -19,3 +20,8 @@ export interface PlayerCharacterStore {
 	getAll(): PlayerCharacter[];
 	save(characters: PlayerCharacter[]): void;
 }
+
+export interface UndoRedoStore {
+	get(): UndoRedoState;
+	save(state: UndoRedoState): void;
+}