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/contexts/encounter-context.tsx

@@ -1,5 +1,6 @@
 import { createContext, type ReactNode, useContext } from "react";
 import { useEncounter } from "../hooks/use-encounter.js";
+import { useUndoRedoShortcuts } from "../hooks/use-undo-redo-shortcuts.js";
 
 type EncounterContextValue = ReturnType<typeof useEncounter>;
 
@@ -7,6 +8,7 @@ const EncounterContext = createContext<EncounterContextValue | null>(null);
 
 export function EncounterProvider({ children }: { children: ReactNode }) {
 	const value = useEncounter();
+	useUndoRedoShortcuts(value.undo, value.redo, value.canUndo, value.canRedo);
 	return (
 		<EncounterContext.Provider value={value}>
 			{children}