Add JSON import/export for full encounter state

Export and import encounter, undo/redo history, and player characters
as a downloadable .json file. Export/import actions are in the action
bar overflow menu. Import validates using existing rehydration functions
and shows a confirmation dialog when replacing a non-empty encounter.

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

apps/web/src/hooks/use-encounter.ts

@@ -444,6 +444,7 @@ export function useEncounter() {
 
 	return {
 		encounter,
+		undoRedoState,
 		events,
 		isEmpty,
 		hasTempHp,
@@ -469,6 +470,8 @@ export function useEncounter() {
 		addFromPlayerCharacter,
 		undo: undoAction,
 		redo: redoAction,
+		setEncounter,
+		setUndoRedoState,
 		makeStore,
 		withUndo,
 	} as const;